of Climate Change in Mesoamerica

Transcription

1 Technical Series Technical Report No. 383 ABC of Climate Change in Mesoamerica Miguel Cifuentes Jara Tropical Agriculture Research and higher Education Center (CATIE) Climate Change Program Turrialba, Costa Rica, 2010

2 The Tropical Agricultural Research and Higher Education Center (CATIE) is a regional center dedicated to research and graduate education in agriculture and the management, conservation and sustainable use of natural resources. Its members include the Inter-American Institute for Cooperation on Agriculture (IICA), Belize, Bolivia, Colombia, Costa Rica, the Dominican Republic, El Salvador, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Venezuela and Spain. Tropical Agriculture Research and higher Education Center, CATIE, 2010 ISBN Credits Technical editors Enric Aguilar, Ph.D. Climate Change Research Group Geography Department University Rovira i Virgili de Tarragona Av. Catalunya, , Tarragona Spain Víctor Orlando Magaña Rueda, Ph.D. Center for Atmospheric Sciences Universidad Nacional Autónoma de México Ciudad Universitaria Mexico City Mexico Editor Elizabeth Mora Copy editors Joselyne Hoffmann Cynthia Mora Designer Rocío Jiménez Salas Translator Christina Feeny

3 Contents Introduction Key concepts of global climate change Climate and the greenhouse effect Climate change Natural climate variability Planetary movements Solar radiation Volcanic eruptions Human influence on climate Greenhouse gases (GHG) Aerosols The unequivocal human action Evidence of climate change Temperature Precipitation Changes in the oceans Ice and snow cover Extreme events Climate scenarios IPCC carbon emissions scenarios The importance of considering several scenarios Projections of future climate change Areas of uncertainty in the predictions Climate in Mesoamerica Historical climate patterns Precipitation

4 ABC of Climate Change in Mesoamerica Temperature Changes observed in climate variables Climate scenarios for Mesoamerica Expected changes in temperature and precipitation Effects of climate change on Mesoamerica Water resources Biodiversity Climate change severity index Terrestrial ecosystems Aquatic ecosystems Freshwater systems Mangrove forests and coral reefs Coastal zones Fisheries Agriculture and cattle ranching Generalities of the sector Changes in production Human health Disasters Other sectors Bibliography Annex Annex

5 Introduction Introduction Human activities have brought about changes in the natural functioning of the Earth s climate system. Potential effects are varied and affect all areas of human endeavor. The scale of the changes and a limited capacity for response make Mesoamerica the region most vulnerable to climate change in the entire tropical region. In the face of this threat, it is essential to have access to high quality information to better understand the scope of the potential effects of climate change and design strategies to address them. The purpose of this document is to provide up-to-date scientific information to support the formulation of the Regional Strategy on Climate Change for Central America and the Dominican Republic. The strategy aims to guide the actions of different sectors, institutions and organizations (governmental, private and civil society) to respond more effectively to the impacts and challenges of climate change. It will also help the countries of the region to position themselves in the global process of discussion and negotiation on climate change. This document consists of three main parts. The first contains a detailed description of the processes that generate climate on Earth, the role played by human activities in influencing climate, the scientific evidence related to climate change and an analysis of climate scenarios. The second part of the document contains a summary of historical climate patterns in the region, the changes observed in recent decades and the predictions for future climate. The third section offers a synthesis of the potential impacts of climate change on those sectors of society which, according to the Intergovernmental Panel on Climate Change (IPCC), would be most affected by climate change. 5

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7 Key concepts of global climate change 1 Key concepts of global climate change Climate and the greenhouse effect Climate is defined as the set of states and changes in atmospheric conditions observed in a given area over a period of at least 30 years. Average conditions, together with their variability, and extreme events of precipitation, temperature, wind, atmospheric pressure, etc. are all expressions of a region s climate. The climate of an area is a dynamic phenomenon subject to variability and change. Solar radiation is the main source of energy for the planet s climate system. More specifically, the balance (known as radiative balance ) between the energy received by our planet from the sun, and energy that it re-emits, is the main mechanism that determines the Earth s climate. To balance the amount of incoming energy absorbed, the Earth must radiate approximately the same amount of energy back to space. This occurs in the form of long wave energy, also known as thermal radiation. Approximately 30% of the solar energy reaching our planet is reflected directly back into space by the highest layers of the atmosphere and by surfaces with a high albedo 1, such as those covered with ice and snow. The remaining two-thirds of the incident energy are absorbed by Earth s surface and by the atmosphere. Some trace gases in the atmosphere (carbon dioxide, methane, among others) absorb a large amount of thermal radiation emitted by the surface of the planet and radiate it back to Earth again. This natural 1 Albedo is a fraction of solar radiation reflected by a surface or an object, often expressed as a percentage. 7

8 ABC of Climate Change in Mesoamerica phenomenon is known as the greenhouse effect and results in the warming of the planet s surface (Figure 1). If the natural greenhouse effect were not in place, the temperature of Earth s surface would be -18 ºC and would fluctuate widely between day and night. Therefore, the natural greenhouse effect makes life as we know it possible on Earth. Atmospheric trace gases that directly contribute to the greenhouse effect are commonly known as greenhouse gases or GHG. The main greenhouses gases that contribute to global warming are water vapor and carbon dioxide (CO 2 ). Other important GHG are methane (CH 4 ), nitrous oxide (N 2 O), ozone (O 3 ), among others. Human activities have increased the concentrations of carbon dioxide, methane, chlorofluorocarbons, etc. in the atmosphere, further intensifying the greenhouse effect and thereby increasing Earth s surface temperature. Climate change The climate system changes over time under the influence of its own internal mechanisms (such as El Niño/Southern Oscillation) and also because of external factors known as natural drivers or forcings. Some of the most important external natural forcings affecting climate are variations in solar activity, planetary movements, volcanic eruptions and changes in the composition of the atmosphere. Recently, scientists have determined that human activities more specifically, increases in concentrations of greenhouse gases in the atmosphere have become a dominant external forcing on the climate, being responsible for most of the warming observed in the last 50 years. This phenomenon, is popularly known as global warming, or more broadly as climate change when other effects are considered. The IPCC definition of climate change does not distinguish between natural and anthropogenic causes of climate change, whereas the 8

9 Key concepts of global climate change Figure 1. Idealized model of the greenhouse effect. From Solomon et al. (2007). definition of the United Nations Framework Convention on Climate Change (1992) describes this process as a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability over comparable time periods. Natural climate variability Planetary movements Long before human presence on Earth, the planet s energy balance, and therefore its climate, was affected by various natural causes. For example, there is strong evidence showing that ice ages occur periodically and that these are linked to variations in Earth s orbit. These changes are known as Milankovitch cycles (Figure 2), which are regular variations (in the order of hundreds of thousands of years) in the eccentricity of Earth s orbit around the Sun, and changes in 9

10 ABC of Climate Change in Mesoamerica Earth s obliquity 2 and precession 3. Such variations in the planet s movements alter the amount of incoming solar radiation received by the planet at different latitudes, producing drastic changes in the global climate. Solar radiation Another likely cause of climate change is the variation in the amount of energy produced by the Sun. For example, sunspot observations as well as data from isotopes generated by cosmic radiation, show that solar radiation varies (by nearly 0.1%) in short 11-year cycles and also over much longer periods. Figure 2. Diagram of Earth s orbital changes (Milankovitch cycles) that drive ice age cycles. T denotes changes in the tilt (or obliquity) of the Earth s axis and E refers to changes in the eccentricity of the orbit. P denotes precession, or changes in the direction of the axis tilt at a given point of the orbit. Taken from Solomon et al. (2007). 2 Obliquity refers to the tilt of the Earth s rotational axis, with respect to the plane of its orbit around the Sun. 3 Precession refers to the oscillatory movement, around its axis, exhibited by a rotating body. 10

11 Key concepts of global climate change However, the impact of these periodic changes in solar radiation is still unknown. In theory, changes in solar activity directly affect the climate by altering the amount of energy reaching the planet. Solar radiation also affects the concentrations of some greenhouse gases, such as stratospheric ozone. Volcanic eruptions Catastrophic volcanic eruptions have the capacity to reduce global temperature. When an explosive volcanic eruption occurs, enormous quantities of ash, dust and sulfate aerosols are expelled into the stratosphere. These materials form a kind of natural barrier or shield that reflects solar radiation back to space before it reaches the planet s surface, causing temperatures to decrease. However, this cooling effect is of short duration (2 to 3 years), as was the case after the eruptions of Mount Agung in Bali in 1963, Chichón Volcano in Mexico in 1983, and Mount Pinatubo in the Philippines in Even the soot expelled by the burning oil wells in Kuwait appears to have reduced the amount of solar radiation reaching the Earth s surface at that time. As a result, the planet s temperature decreased slightly for a few months. Human influence on climate Climate changes produced by humans are mainly the result of increases in concentrations of greenhouse gases in the atmosphere (Figure 3), and of changes in the amounts of aerosols (small particles) that float in the atmosphere. These changes are capable of altering the planet s energy balance and increasing or decreasing the temperature. Greenhouse gases (GHG) Human activity results in the emission of several greenhouse gases (GHG), the most important being carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O) and halocarbons (gases that contain fluorine, chlorine and bromine). The amount of GHG produced by 11

12 ABC of Climate Change in Mesoamerica human activities increased by 70% between 1970 and Current atmospheric concentrations of CO 2, CH 4 and N 2 O greatly exceed pre-industrial values. In 2005, concentrations of CO 2 and CH 4 greatly exceeded the natural range over the last 650,000 years. Higher concentrations of these gases in the atmosphere result in an increase in temperature. Carbon dioxide is the most important greenhouse gas due to the enormous amounts of it released into the atmosphere. Since the start of the industrial revolution and the development of an economy based on fossil fuels burning, and up to 1970, CO 2 has increased by 100 ppmv (parts per million by volume). Between 1970 and 2004, annual CO 2 emissions increased by 80%. Approximately 75% of the CO 2 increase is attributed to the use of fossil fuels (mainly in the transport sector) and to cement production. The remaining CO 2 comes from deforestation and land use changes, which also release CO 2 and decrease the amount of this gas that could be sequestered by forests. The current rates of increase in carbon dioxide (along with those of nitrous oxide and methane) are unprecedented, at least in the last 16,000 years. Current concentrations of CO 2 in the atmosphere have reached 384 ppm (parts per million), and show no signs of decreasing or stabilizing (Figure 3). This value exceeds the range of the natural variability known in the last 650,000 years. CH 4 is the second most important greenhouse gas. It is released primarily as a result of anaerobic processes in agriculture, the production of natural gas and waste treatment in landfills. Concentrations of CH 4 in the atmosphere have risen from 715 ppm prior to the industrial revolution, to 1774 ppm in 2005 (Figure 3). N 2 O is emitted due to fertilizer use and the burning of fossil fuels, although it is also released through natural processes. Atmospheric N 2 O increased from 270 to 319 ppm between 1750 and 2005 (Figure 3). Finally, chlorofluorocarbons (CFCs, a type of halocarbons) are completely synthetic gases introduced by humans, and do not occur naturally. 12

13 Key concepts of global climate change Aerosols Aerosols are very small particles (between 0.07 and 20 μm, depending on their origin) suspended in the atmosphere. Aerosols vary greatly in terms of their concentration, chemical composition and size, and may be of natural or anthropogenic origin. The burning of fossil fuels and biomass has increased the amount of sulfur aerosols, organic compounds and soot (also called black carbon ) in the atmosphere. Mining and other industrial processes release additional amounts of aerosols and dust. In general, aerosols produce a cooling of earth s temperature because they reflect solar radiation. For example, the accelerated industrial development after the Second World War increased atmospheric pollution in the Northern Hemisphere, contributing to a decrease in temperature between 1940 and 1970 approximately (Figures 4 and 6) Figure 3. Concentrations of greenhouse gases in the last 2,000 years. Increases since 1750 are attributed to human activities during the industrial era. Concentration units are measured in parts per million (ppm) or parts per billion (ppb). Adapted from Solomon et al. (2007). 13

14 ABC of Climate Change in Mesoamerica The unequivocal human action It is very likely (Annex 2 contains specific definitions regarding IPCC guidelines on uncertainty) that the increase in atmospheric greenhouse gases concentrations since the beginning of the industrial age (around 1750) has had a net warming effect on temperature. Numerous experiments have been carried out using different climate models to determine the probable causes of climate changes that occurred in the 20 th century. The results of these experiments show that natural forcings (solar radiation, aerosols from volcanic eruptions, etc.) are not sufficient to explain the trend toward rising temperatures on Earth. These trends can only be replicated by including human influence in the models (Figure 4). Furthermore, human influence considerably exceeds the intensity of any natural forcing that could otherwise control the climate (Figure 5). It is estimated that even if humans were to drastically reduce their greenhouse gas emissions, global warming would proceed more rapidly than during the last 10,000 years. This is because the influence of greenhouse gases on the planet s energy balance persists during a very Figure 4. Changes in surface temperature ( C) relative to the mean value, from one decade to another, from 1906 to The black line shows changes in observed temperatures y colored bands show the range covered by 90% of the simulations from recent models. The red band shows simulations that include natural and human factors, while the blue bands show simulations that include only natural factors. Adapted from Solomon et al. (2007). 14

15 Key concepts of global climate change long time. In other words, there is sufficient scientific evidence to affirm that the global warming observed recently is the result of human action. Figure 5. Main components of radiative forcing of climate change. Values represent the forcings in 2005 relative to the start of the industrial era (around 1750). Positive forcings lead to a warming of the climate and negative forcings to a cooling. Error bars represent the range of uncertainty for each forcing. Taken from Solomon et al. (2007). 15

16 ABC of Climate Change in Mesoamerica Evidence of climate change The warming of oceans and of the land surface, changes in the distribution and intensity of precipitation, rising sea levels, the melting of glaciers, the displacement of sea ice in the Arctic and the shrinking snow cover in the Northern Hemisphere are all signs that confirm the warming of the planet s surface. However, the observed changes do not occur uniformly all over the world. For example, due to the influence of local factors, there are some areas of the world where the temperature has actually decreased, even though the global average is increasing. This is consistent with climate trends on a smaller spatial scale and is not sufficient to negate the warming at global level. Temperature Between 1906 and 2005 global surface temperature of the Earth increased by 0.74 ºC (Figure 6a). Average temperatures in the Northern Hemisphere during the second half of the 20 th century were likely the highest of the last 1,300 years. However, the increase has not been uniform around the world, neither in spatial nor in temporal terms. For example, warming has been greater over land than over the oceans, particularly since the 1970s. Seasonally, warming has been slightly greater in the winter hemisphere (also called the dark pole or pole in shadow) and in the high northern latitudes. However, in some parts of the world, such as the northern part of the North Atlantic, temperatures have decreased. During the last century, warming occurred in two phases, with an accelerating rate in the last 25 years: between 1910 and 1940 the average global temperature increased by 0.35 ºC, and from the1970s it rose by 0.55 ºC (Figure 6a). Eleven of the 12 warmest years since records began have occurred since Consistent with this warming, a decrease in the number of very cold days and nights has been observed. Furthermore, the duration of the ice-free season has increased in most middle and high latitude regions of both hemispheres. In the northern hemisphere, this translates into an earlier start to spring. 16

17 Key concepts of global climate change Precipitation Precipitation shows greater spatial and temporal variability than temperature. Changes observed in some regions are dominated by long-term variations, though trends were not evident during the 20 th century. During this same period, annual precipitation increased significantly in eastern parts of North and South America, northern Figure 6. Changes in: a) global average surface temperature; b) global average sea level and c) Northern Hemisphere snow cover for March-April. From IPCC (2007). 17

18 ABC of Climate Change in Mesoamerica Europe and northern and central Asia. By contrast, the Sahel, the Mediterranean, southern Africa and parts of south Asia are now drier than at the start of the 20 th century (Figure 7). In northern regions, precipitation in the form of rain is now more common that in the form of snow. Changes in the oceans Warming has been most evident in parts of the middle and low latitudes, particularly in tropical oceans. Since 1961, the oceans have absorbed more than 80% of the heat added to the climate system. This has caused an increase in the average global temperature of the ocean to a depth of at least 3,000 m, with the consequent rise in sea level. Thermal expansion of seawater and the melting of ice, both processes due to the increased global temperatures, contribute Figure 7. Changes in annual precipitation are not homogeneous around the world. In general, average precipitation during the 20 th century increased in continents outside the tropics, but decreased in arid regions of Africa and South America. Yellow circles represent decreases in precipitation and green circles represent increases. The size of the circle represents the scale (%) of the change. Adapted from IPCC (2001). 18

19 Key concepts of global climate change to rising sea levels. Thermal expansion has contributed 57% to the observed increase. Retreating glaciers, ice caps, and ice sheets are responsible for the remaining increase, at an annual rate of 1.2 ± 0.4 mm, between 1993 and However, sea level has not risen uniformly around the world due to variations in the temperature changes in the oceans, the salinity of the water and oceanic circulation patterns. Sea level has gradually risen since the end of the nineteenth century, and it continues rising even more rapidly (Figure 6b). During the 20 th century, the average rate of sea level increase was 1.7 mm per year. It is also likely that the rate of extreme sea levels has increased around the world since Global sea level is projected to continue rising during the 21 st century, and to do so at a faster rate than between 1961 and Thermal expansion is projected to contribute most heavily to average sea level increases for the next 100 years, at least, particularly if greenhouse gas concentrations are not stabilized. Ice and snow cover In the northern Hemisphere, springtime snow cover has been declining by 2% per decade since 1966 (Figure 6c). Furthermore, the snow is melting earlier in spring. There has been a widespread decline in mountain glaciers and snow cover in both hemispheres, while annual Artic sea ice extent has been shrinking at an average rate of almost 3% per decade. The decrease in the area of sea ice exceeds 7% per decade. The area of permafrost and seasonally frozen ground, as well as the ice in rivers and lakes, has also decreased. Extreme events Extreme events refer to maximum or minimum values of a particular variable, or to infrequent climatic events of great intensity (for example, storms, droughts, heat waves). In the last 50 years, the number of cold nights has decreased and the number of warm nights has increased. Maximum and minimum temperatures have also 19

20 ABC of Climate Change in Mesoamerica increased (Figure 8). The number of frost-free days has increased as the temperature has risen in middle latitudes. It is likely that heat waves are now more frequent in most land areas. It is to be expected that a warmer climate will increase the risk of drought in places where it does not rain, and increase the risk of flooding in areas where it does rain. The distribution and timing of droughts and floods is most profoundly affected by the cycle of El Niño events, particularly in the tropics and in many parts of the midlatitudes of the Pacific Rim countries. The intensity of precipitation and the risks of intense rainfall and snowfall increased during the 20 th century due to an approximate 5% increase in atmospheric water vapor. As a result, during the last 50 years more intense precipitations have been observed in warm climates, even in places where overall annual precipitation is decreasing. Figure 8. Trends observed (in days per decade) from 1951 to 2003 in the frequency of extreme temperatures, relative to mean values for period between 1961 and 1990: a) cold nights, b) cold days, c) warm nights and d) warm days. The red line shows the decadal variations. Adapted from Alexander et al. (2006). 20

21 Key concepts of global climate change This means that the seasonality of rainfall is now more marked. Very dry land areas across the globe have more than doubled in extent since the 1970s, and droughts have become more common in many regions of the planet. It is also very likely that even stronger events will occur as overall levels of precipitation increase. The number of category 4 and 5 hurricanes has increased by about 75% since The largest increases have been observed in the North Pacific, Indian and Southwest Pacific Oceans. In the North Atlantic, the number of hurricanes was also above average in 9 of the 11 years during the period from 1996 to However, the detection of long-term trends in cyclonic activity is not yet very reliable. Climate scenarios Projections of future climate change have a certain level of uncertainty due to the changing nature of the climate and to the difficulty in determining future levels of GHG emissions. Concentrations of GHG depend on many assumptions and factors with varying degrees of uncertainty, such as population growth, development and use of alternative energies, technological and economic development, and human policies and attitudes toward the environment. For these reasons, the different scenarios used contemplate different ranges of these factors to investigate the potential consequences of anthropogenic climate change. A climate scenario is defined as a plausible and generally simplified representation of a possible future climate, based on an understanding of how the climate works and of the different factors that influence it. Scenarios are typically constructed as input to evaluate the possible impacts of climate change on natural and social systems. 21

22 ABC of Climate Change in Mesoamerica IPCC carbon emissions scenarios The IPCC Special Report on Emissions Scenarios (SRES; see IPCC, 2000) contains 40 different scenarios, grouped into four families (Table 1) that explore alternative forms of development. The scenarios incorporate demographic, social, economic, technological, and environmental factors, together with the resulting greenhouse gas emissions, to draw some conclusions about future climate change. The main rationale behind these scenarios is that societies have the option of either working together to resolve global problems through joint and comprehensive solutions, or remaining isolated and trying to resolve their problems independently. Furthermore, development goals may be aimed at increasing human wealth or at conserving the environment (Figure 9). Figure 9. Conceptual framework of IPCC families of climate change scenarios. The horizontal axis represents ways of adapting to problems while the vertical axis represents the type of development. Adapted from Palma Grayeb et al. (2007) and Anderson et al. (2008). 22

23 Key concepts of global climate change Table 1. Characteristics of IPCC families of climate change scenarios. Family Number of scenarios Characteristics A1 17 A2 6 B1 9 B2 8 Rapid economic growth, low rate of population growth, and rapid shift towards more efficient technologies. Convergence between regions. Differences in personal income significantly reduced. This family is divided into three groups based on the energy system used: intensive use use of fossil fuels (A1F), use of non-fossil fuels (A1T), and balance between different sources (A1B). A very heterogeneous, self-sufficient world that maintains local identities. Population growth rates converge slowly, which results in high population growth. Per capita economic growth is slower and more fragmented than in other families. A convergent world, with low population growth and rapid changes in economic structures. Shift toward an economy based on services and information technology. Less intensive use of materials, introduction of cleaner and more efficient technologies. Emphasis on global solutions to promote environmental, economic and social sustainability and greater equity. A world emphasizing local solutions to environmental, social, and economic sustainability. Population growth and economic development are moderate. Technological change is less rapid but more diverse than in B1 and A1. This family focuses on environmental protection and social equity, but at the regional and local levels. Source: IPCC (2000). 23

24 ABC of Climate Change in Mesoamerica Other than those already existing, IPCC scenarios do not explicitly contemplate climate policies that focus directly on reducing greenhouse gas emissions and maximizing the size of CO 2 sinks. Instead, the idea is that the scenarios will serve as a reference for analyzing the potential consequences of implementing additional policies. All scenarios are considered equally valid and likely. This leaves the door open so political discussions regarding possible courses of action in response to climate change can take place. The importance of considering several scenarios Comparing groups of similar models, or making comparisons between models with different structures, is useful to quantify the probabilistic aspect of the scenarios. It is also necessary to construct various future climate scenarios to quantify the uncertainty of the estimates. In terms of policies, instead of deciding whether a specific model is most representative of certain future conditions, considering several models enables us to expand our options for developing a broader range of adaptation alternatives. For this reason, the IPCC recommends that at least two families of scenarios with a wide variety of assumptions be considered in any analysis of climate change. In the last simulations of global climate change carried out for IPCC the B1, A1B and A2 scenarios were used. These are interpreted as possible low, medium and high levels of emissions. In the Mesoamerican region, the A2 and B2 scenarios are most commonly used. Projections of future climate change If current climate change mitigation policies and sustainable development practices are maintained, greenhouse gas emissions will continue to increase in the coming decades. As a result, global warming will intensify during the 21 st century, with climate changes very likely greater than those experienced in the 20 th century. The warming projected for the 21 st century would have a geographic distribution similar to that observed until now. 24

25 Key concepts of global climate change According to IPCC projections, the global average temperature increase observed between 1990 and 2005 (0.15 and 0.30 ºC per decade) will remain approximately the same during the next 20 years. This trend would not change even if concentrations of all greenhouse gases and aerosols were to remain at constant levels similar to those of Despite the fact that the exact ranges of temperature change vary slightly between climate scenarios, all IPCC scenarios show temperature increases (Table 2); up to 6 ºC in the most extreme estimate. It is very unlikely that the temperature increase will be less than 1.5 ºC. The extent of the area covered with snow and sea ice will continue to shrink. It is very likely that extreme temperatures, heat waves and intense precipitations will become more frequent. It is likely that future tropical cyclones will be more intense due to higher sea surface temperature. Trajectories of extra-tropical storms are projected to shift toward the poles. It is very likely that precipitation will increase in high latitudes, while decreasing by up to 20% in subtropical regions. Past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rises for more than a millennium. Even if all radiative forcings are stabilized and maintained constant by 2100, we would still expect to see an increase of about 0.5 ºC in global average temperature up to Thermal expansion of the oceans would continue for many centuries, due to the time required to transport heat down to the deepest layers of the ocean. Sea level rise is projected to reach 0.3 to 0.8 m, relative to the level, towards the year 2300 (see Table 2 for estimates up to 2100). If the Greenland Ice Sheet were to disappear, sea level would increase by up to 7 m. This value is similar to the sea level estimated for the last interglacial period, some 125,000 years ago. 25

26 ABC of Climate Change in Mesoamerica Table 2. Average range of temperature (ºC) and sea level (m) increase for the main IPPC climate scenarios. Case Temperature Increase* Sea level rise Constant GHG concentrations Year Not available Scenario B Scenario A1T Scenario B Scenario A1B Scenario A Scenario A1FI * Likely temperature and sea level increases for relative to Adapted from IPCC (2007). Areas of uncertainty in the predictions Although our knowledge of the global climate system continues to expand rapidly and significantly, there are still uncertainties 4 regarding some of the observed climate changes. These uncertainties do not necessarily negate or invalidate the predictions made. It is simply that there are certain areas of scientific knowledge in which the driving mechanisms are not fully understood. Some of the main areas of uncertainty are mentioned below. Analyzing and monitoring observed changes in extreme events (droughts, hurricanes, frequency and intensity of precipitation, etc.) is more complex than with average climate patterns, since these require longer time series and a range of spatial and temporal scales. The adaptive capacity of some natural and human systems also makes it difficult to detect the effects of climate change and its driving forces. 4 See Annex 2 for a description of the IPCC s treatment of uncertainty. 26

27 Key concepts of global climate change Although most of the climate change models currently used are consistent in their simulations of global-level patterns, there are still difficulties in simulating certain changes (such as precipitation) at regional levels. At these smaller scales, changes in land use or specific pollution problems make it more complicated to detect the effects of anthropogenic warming on natural systems. The intensity of climate feedback processes such as ocean heat uptake, the role played by clouds and the carbon cycle have yet to be quantified with greater certainty. Similarly, the full impacts of aerosols on cloud and precipitation dynamics remain uncertain. The scale of future sea level rise is still unknown (especially its upper limit) due to uncertainty surrounding estimates of ice-sheet loss in Greenland and the Arctic, and the process of heat distribution in the oceans. 27

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29 Climate in Mesoamerica 2 Climate in Mesoamerica Historical climate patterns In Mesoamerica, precipitation and temperature exhibit well-defined annual patterns, modified periodically by fluctuations in the temperature of the surrounding oceans and by the El Niño/La Niña cycles, the Pacific Decadal Oscillation (PDO). In general, the interactions between the trade winds from the east and the region s orographic complexity determine the differentiated precipitation patterns of the region s Caribbean (windward) and Pacific (leeward) slopes. The effects of the rain shadow created by the mountain systems generally mean that the Caribbean slope is rainy practically all year round, while the Pacific slope is characterized by a prolonged dry season (Figure 10, compare stations of Limón and Puerto Lempira on the Atlantic vs. La Unión and Liberia on the Pacific). The southern part of Central America is rainier than the north. Precipitation On the Pacific side of Mesoamerica, precipitation is characterized by a prolonged dry season lasting approximately from November until April or May, and a wet season during the rest of the year. The increased intensity of the trade winds in July produces a peak of precipitation in most parts of the Caribbean slope of Central America and southern Mexico (Figure 10). Due to prevailing trade winds from the east in the region, any rise in the surface temperature of the ocean east of the isthmus causes an increase in precipitation. In contrast, when the temperature of the ocean s surface decreases, precipitation declines by up to 40% during the months of July/August 29

30 ABC of Climate Change in Mesoamerica (during the mid-summer dry period commonly known as veranillo or canícula) in the Pacific slope (see the Liberia and Palmar Sur stations in Figure 10). The veranillo is more pronounced on the western side of Central America, in the Yucatán Peninsula, and in eastern Mexico. However, this phenomenon is practically absent in western Mexico, southern Belize, the south east of Honduras, eastern Nicaragua and Costa Rica, and the north east of Panama. Severe droughts on the Pacific coast are associated with the El Niño phenomenon (an increase in the surface seawater temperature of the equatorial Pacific that generates anomalies in the planet s atmospheric circulation). At the same time, masses of cold air from North America during the winter months, and the trade winds between July and August, produce intense rains that cause flooding on Central Figure 10. Topography and monthly precipitation for selected meteorological stations (location shown with + ) in Central America and southern Mexico. Inset graphs show monthly precipitation (vertical bars) for each station. Taken from Magaña et al. (1999). 30

31 Climate in Mesoamerica America s Caribbean slope. The regions most affected by these conditions are the northern coast of Honduras and the eastern coasts of Nicaragua, Costa Rica and Panama. The northern coast of Honduras and Belize are the areas most susceptible to the direct impact of hurricanes, although the coast of Nicaragua has also suffered their effects in recent decades. Temperature Temperature is strongly related to the temperatures of the Pacific Ocean, including patterns linked to El Niño events. The temperature regimen is also closely related to the annual precipitation cycle. Daily temperatures reach their maximum value before the start of the rainy season and fall around January. Minimum temperatures show a different pattern: the highest values are observed in July (when increased cloud cover decreases radiative cooling) and the lowest values during the Northern Hemisphere winter. Changes observed in climate variables Central America is considered to be the main hot spot for climate change in the tropics (Figure 11). An analysis of temperature and precipitation data from 105 meteorological stations located throughout the Mesoamerican region and in the northern part of South America (Aguilar et al., 2005) show many changes in the extreme values of these variables during the last 40 years. On a regional scale, temperature indices showed significant variations throughout the region during the period between 1961 and 2003 (Table 3). The annual percentage of warm days and nights increased by 2.5% and 1.7% per decade, respectively. At the same time, the number of cold nights and cold days decreased by -2.2 and -2.4% per decade (Table 3). Temperature extremes increased by between 0.2 and 0.3 ºC per decade. The duration of the periods of consecutive cold days also decreased. 31

32 ABC of Climate Change in Mesoamerica Figure 11. The Regional Climate Change Index (RCCI) for 26 land regions of the world, calculated on the basis of 20 General Circulation Models and 3 IPCC emission scenarios. The size of the circles represents the scale of the changes in temperature and precipitation indices. Taken from Giorgi (2006). During the last 45 years, no decrease in annual precipitation has been observed in the region, though there has been a slight increase in its intensity. Furthermore, the number of consecutive dry days has increased. In other words, precipitation patterns have changed so that now it rains during a shorter period of time, but does so more intensely, with obvious impacts on agricultural production, soil conservation, floods, water availability, etc. While most of the meteorological stations analyzed show positive trends (increased precipitation), overall average annual precipitation in the region and the number of consecutive wet days do not show significant changes (Table 3). This is probably due to the limited time periods covered by the data series and to major annual variations in precipitation. Furthermore, the heterogeneity of the precipitation patterns throughout the region (Figure 12) makes it difficult to identify a clear trend for the area as a whole. For example, the number of 32

33 Climate in Mesoamerica Table 3. Trends in regional temperature and precipitation indices for the period Index Units Temperature Trend (units/decade) Warm days % of days 2.5 Warm nights % of days 1.7 Cold days % of days -2.2 Cold nights % of days -2.4 Daily temperature range ºC 0.1 Highest maximum temperature ºC 0.3 Lowest maximum temperature ºC 0.3 Highest minimum temperature ºC 0.2 Lowest minimum temperature ºC 0.3 Duration of cold period number of days -2.2 Duration of hot period number of days 0.6 Precipitation Total annual precipitation mm 8.7 Simple index of daily intensity mm 0.3 Very wet days mm 18.1 Extremely wet days mm 10.3 Maximum precipitation in 1 day mm 2.6 Maximum precipitation in 5 days mm 3.5 Days of strong precipitation number of days -0.1 Days of very strong precipitation number of days 0.1 Consecutive dry days number of days 0.4 Consecutive wet days number of days -0.1 Values in bold are statistically significant at 5%. Adapted from Aguilar et al. (2005). 33

34 ABC of Climate Change in Mesoamerica consecutive dry days decreased in central and southern parts of the region, but increased in northern Mexico and the Caribbean. However, extreme precipitation indices have increased significantly (Table 3) and are strongly and positively correlated with the temperature of the tropical Atlantic Ocean. The latter indicates that prolonged rainy seasons are related to the warm waters in that oceanic basin. The trend over the last 40 years suggests a strengthening of the hydrological cycle throughout the region, with more rain produced by extreme events and greater average precipitation per episode. This trend is expected to continue in the future, possibly resulting Figure 12. Temporal trends in (a) the percentage of warm days, (b) the percentage of cold days and (c) total annual precipitation for the period Red triangles (with the upward pointing apex) represent an increase, and blue triangles (with the downward pointing apex) represent a decrease in the variable. The large triangles represent statistically significant trends, while the small triangles represent non-significant trends. Adapted from Aguilar et al. (2005). 34

35 Climate in Mesoamerica in a greater frequency or intensity of extreme events (floods and/ or droughts). This does not appear to be linked to El Niño. Despite the fact that recent hurricanes have caused extensive damage in the region, it has not been possible to determine with any certainty whether in future these will become more frequent and intense in the Caribbean. Climate scenarios for Mesoamerica The most recent climate scenarios for the region use data generated by the Worldclim 5 project (Hijmans et al., 2005). These scenarios complement the work carried out in the region since the 1990s. Most of the recent climate models for the isthmus underestimate the amount of precipitation in Central America (by up to 60%), but consistently replicate the seasonality of the region s climate, including the veranillo (Rausher et al., 2008). Although IPCC models consider a wide range of very complex interactions between aquatic, terrestrial, and atmospheric components, and their capacity to replicate climate conditions is recognized, their resolution is not the most appropriate for evaluating the effects at the regional or country level. For this, it is necessary to reduce the scale (through a process known as downscaling ) and increase the resolution of the data (STARDEX, 2009; Figure 13). The reference period for climate data is Changes in temperature and precipitation were calculated for the time horizons of 2020, 2050 and These horizons are generic names for the periods , and , respectively. IPCC B2 and A2 scenarios (Table 1) were selected as examples of a favorable scenario and an unfavorable scenario, respectively. In addition, a Climate Change Severity Index (CCSI) was developed, details of which are included in the section on effects of climate change in this document. 5 Last visit 10/23/

36 ABC of Climate Change in Mesoamerica Figure 13. Downscaling and increase of the spatial resolution (from 400 km to 12 km) for a temperature change model for Mesoamerica. Adapted from Anderson et al. (2008). Expected changes in temperature and precipitation The global average surface temperature of the planet is expected to increase between 1.4 and 5.8 ºC up to Consistent with this change, temperatures are expected to rise throughout the Mesoamerican region. However, predictions differ regarding the scale, direction (increase or reduction) and location of the changes in precipitation. Despite this uncertainty, in general, the number of dry days is expected to increase, along with the frequency of more intense precipitations and extreme events such as storms and floods. Future climate changes may possibly be due to changes in the surface temperature of the ocean, the displacement of the inter-tropical convergence zone, the expansion and intensification of the high pressure zones in the north Atlantic, and greater temperature contrasts between the continental mass and the ocean. 36

37 Climate in Mesoamerica The models project a regional increase of 1 to 2 ºC for Other models predict that the temperature would be between 2 and 4 ºC higher in 2080 (for scenarios B2 and A2, respectively). In general, an overall increase in temperature is projected throughout the region, with the extreme north of the region being affected by a greater temperature increase than the extreme south. Toward 2080, for scenario A2, the temperature could rise by as much as 6.5 ºC in the far north of Mesoamerica, in areas around Belize, Peten, and the border between Guatemala and Mexico. In the most favorable scenario, the temperature of that same area would rise by 4 ºC for The area to the south of the Nicaraguan-Costa Rican border would experience a temperature increase of less than 2 ºC under both scenarios (Figure 14). The rest of the region would experience gradual changes between these geographic extremes. Precipitation projections are more heterogeneous, both in spatial and temporal terms. In general, the greater portion of the Mesoamerican region, especially the Pacific coast, will experience a decrease in precipitation toward Under the favorable scenario, the exceptions are the southwest coast of Guatemala and the far south of Panama, where there would be a slight increase in precipitation but only under the favorable scenario. By contrast, under the unfavorable scenario, precipitation in the northeast coast of Honduras, all of Nicaragua, most of Costa Rica and the central and northern portion of Panama would decline by at least 20% (Figure 14, Table 4). The rest of the region would also experience decreased precipitation under this last scenario, although not as severe as the other areas mentioned. Other models (Rausher et al. 2008) predict greater reductions in precipitation in southern Guatemala, El Salvador, Honduras and western Nicaragua. With regard to the spatial distribution of precipitation in the future, there are differences between the recent models. For example, simulations done by SICA et al. (2006) and the results of PRECIS 6, show 6 and eng/datos.html. Last visit 10/23/