Physics in the Current Climate

Transcription

1 Physics in the Current Climate Bethan White; Looking at the type of groups that exist in most UK physics departments wouldn t lead you to believe that physics has a role to play in climate change research. Yet tucked away in earth sciences, geography and the occasional physics department across the country, physicists are making their contribution. The field of climate modelling is one where a physicist s skills are vitally important, combining modelling physical processes with numerical modelling and scientific programming. A climate model is a mathematical model of the interactions of the land, ocean, atmosphere and ice when averaged over daily, seasonal and yearly fluctuations. Not just useful for predicting the weather, studies of climate models can help us to predict what the future climate on Earth may be like. They are a vitally important tool in researching the causes and effects of climate change. A wide range of physics is used to model the climate. This includes (but is not limited to) the interactions of solar radiation with our atmosphere, the thermodynamics of our atmosphere, the fluid dynamics of the atmosphere and oceans, radiative transfer in the atmosphere and the physics of clouds [1]. Putting all these physical processes together in a system is a complicated process and requires a comprehensive knowledge of the physics involved. Some basic physical processes of the geosystem Solar radiation 99.97% of the energy that enters our climate system comes from the Sun [1]. The solar energy reaching the Earth at any point on the surface, F, can be calculated by using the solar constant S and terms for the reflection R and transmission T of the energy, and by introducing a factor of θ to allow the Sun to move position with respect to the zenith: F = S( 1 R) T cosθ Wm -2 (Eqn 1) Solar radiation that reaches the surface interacts with the atmosphere. Some atmospheric gases absorb energy at particular frequencies, having important consequences. Ozone, for example, absorbs strongly in the UV range, acting as a filter which protects plants and animals from the dangers of UV photons. Of the total energy reaching the Earth from the Sun, about 50% reaches the surface, while 30% is reflected back into space by clouds, snow and ice (the albedo), and 20% is absorbed by the atmosphere. The Earth is also in radiative balance; the total amount of energy absorbed is equal to the total amount emitted. The atmosphere The atmosphere is composed mainly of nitrogen, oxygen, water vapour, argon and carbon dioxide. This fluid layer of gas is bound gravitationally to the surface of the

2 rotating Earth. The simplest description is a one-dimensional model. This vertical profile is created by combining the ideal gas law with the equation for hydrostatic equilibrium (the one-to-one relationship between pressure p and height z), giving: mgz RT p = p 0 e Nm -2 (Eqn 2) where p 0 is the pressure at ground level, g is the acceleration due to gravity, R is the gas constant, T is the temperature of the atmosphere at height z and m is the mass of one mole of gas. This expression shows that atmospheric pressure decreases exponentially as the height increases. The atmosphere undergoes circulation, transporting clouds, water, ozone and heat around the world. For a stationary planet we expect air to rise in the region heated by the Sun and fall on the other side of the Earth in the cool region, giving two cells of circulation. If we then allow the planet to rotate slowly, this splits into four cells (one in each hemisphere) known as Hadley cells. However, the Earth undergoes relatively rapid rotation and the Coriolis force causes each of these cells to split into three cells of circulation the Hadley, Ferrell and Polar cells [1], shown in figure 1 [2]. Figure 1 - Idealisation of the Earth's atmospheric circulation, showing the Hadley, Ferrell and Polar cells. The ocean The ocean, like the atmosphere, is a fluid gravitationally bound to the rotating Earth. The direct heating of the Sun is limited to the top mixed layer of water. In deep water the temperature is nearly constant, decreasing slowly to the bed. Salt concentrations have a large effect on the oceanic structure and circulation. Salinity is highest near the surface due to the concentrating effect of evaporation. Again, in the deep ocean it is nearly constant with depth, increasing slowly towards the bed. The circulation of the ocean consists of two parts: a top layer and a deep ocean lower layer. The deep ocean layer contains more than 90% of the mass of the oceans. The thermal and salinity gradients seen in this layer result in density differences and hence circulation; this is known as the thermohaline circulation, shown in figure 2 [3]. This motion is slow and adiabatic. The average of this motion can be determined by balancing the density gradients with the Coriolis force, resulting in geostrophic flow where water moves along isobars [1].

3 Figure 2 - The global oceanic circulation (highly simplified). The red part represents the net transport of warm water in the top 1000m or so of the oceans, and the blue part the net transport of cold water below this level. Near the surface, wind drag and surface waves cause turbulent mixing of the top layer leading to a departure from geostrophic flow. This non-geostrophic layer is called the Ekman layer. It is obvious from this brief overview that a comprehensive study of the Earth s processes could not be made without serious consideration of the physics involved. A global model of the climate must bring all of these processes together into one system. This is no easy task and has a long history. In 1922, L.F. Richardson (a British mathematician and physicist) became the first person to devise a numerical system for modelling the weather. He divided the Earth into a grid of cells and made calculations for each cell (carried out by hand as computers were not then available). These calculations failed to predict the weather, but the method of using grid cells was to be extremely important in developing subsequent models. By the 1960s, computers had advanced significantly and weather forecasters were using 3D models of atmospheric flow to predict the weather several days in advance. However, climate models proved much more difficult to develop and even in the 1980s they were not very sophisticated. Weather models are built from the interactions of atmospheric parameters (temperature, pressure, humidity, wind and clouds) and can ignore factors which do not change much over the short timescale of weather forecasts (such as ocean temperatures). Climate models, which look at much longer timescales, must take all environmental systems into account. Factors such as ocean temperatures, seaice, vegetation and even the topography of the Earth become important. This makes the system much more complex [4]. Types of climate model The zero-dimensional model The simplest possible model is the zero-dimensional radiative equilibrium of the Earth. By balancing the incoming solar radiation with the outgoing energy from the Earth, the Earth s surface is represented as a single point. Its use is limited as it gives the radiative temperature of the Earth rather than the surface temperature. These two temperatures are different and is accounted for by the natural greenhouse effect of the atmosphere.

4 The radiative-convective model This is a one-dimensional representation of the atmosphere which contains only two possible methods of energy transport. Radiative transport can occur both up and down through atmospheric layers. Heat can also be transported upwards by convection. The Earth s surface, as in the zero-dimensional model, is only represented by a single point. However, the surface temperature can be calculated from this type of model, as can the effect of greenhouse gas concentrations. Energy Balance Models The zero-dimensional model can be extended to a one-dimensional model to include the horizontal transport of energy in the atmosphere. The albedo can be included in this model the poles can be ice-covered and cold; the tropics can be warm and free from ice. The lack of a full description of dynamical systems in this type of model means that the transport of energy must be specified; hence its use is again limited. GCMs Global Circulation Models, or Global Climate Models GCMs are three-dimensional models which model the atmosphere and oceans using the equations of fluid motion. The Earth s surface is divided up into a grid of cells and the fluid equations are discretised for each cell. Other processes are parameterised and included, such as convective motion. Current climate models Today s climate models are mostly GCMs. They are much more detailed than the original 1960s models, which included only a simple layered atmosphere with no geography [5]. Separate models have been developed to describe the different components of the global climate. Atmosphere General Circulation Models (AGCMs) These consist of a 3D model of the atmosphere that is coupled to the land surface. Data for the sea-surface temperature and sea-ice coverage is not included in the model and must be input. Because of this, an AGCM cannot be used to predict climate on its own as it cannot model the change in oceanic conditions. They are useful for studying atmospheric processes and the response of the atmosphere to changes in sea-surface temperature. AGCMs can also be coupled to a slab ocean. This models the ocean as a layer of water of constant depth. Changes in sea-surface temperature and sea-ice can be predicted from this. The heat transport in the ocean remains a constant parameter, due to the nature of the model. This is useful for predicting the climate for fixed CO 2 levels but cannot predict the rate of climate change as this is largely controlled by oceanic processes. Ocean General Circulation Models (OGCMs) This is the oceanic equivalent of the AGCM, a 3D model of the ocean and sea-ice. Like the AGCM, it is useful for studying ocean processes alone, but relies on data input for surface air temperature and other atmospheric processes.

5 Coupled Atmosphere-Ocean General Circulation Models (AOGCMs) This is the most complex type of climate model in use today, and is run in institutions such as the Hadley Centre [6]. An AGCM is coupled to an OGCM (sometimes other processes are also included) to enable the future climate, and its rate of change, to be predicted. Other types of model in use include regional climate models (RCMs), atmospheric chemistry models and carbon cycle models [4]. The future lies in refining current physical models, coupling more processes together and running models at higher resolutions. As a society that has been pumping out greenhouse gas emissions for decades, we have been performing an experiment on the Earth without any idea of what the results might be. In order to be prepared for those results, and in order to take action against them, we need a powerful tool such as climate modelling to give us a head start. If we had many Earths, each with different concentrations of greenhouse gases, then we would know the outcome of what we are doing. As it stands, we only have one Earth. Using models to simulate different outcomes is our only way of knowing what might happen. Without physics we couldn t even begin to model the processes that make up our climate, let alone put them all together. What is the role of physics in climate modelling? It is fundamental. References: [1] Elementary Climate Physics, F.W. Taylor, Oxford University Press, 2005 [2] [3] [4] The Rough Guide to Climate Change, Robert Henson, Penguin, 2006 [5] General Circulation Models of the Atmosphere - essay overview.htm [5]