Chapter 1: Overview of the Climate System
1.1. THE CLIMATE SYSTEM AND SOME DEFINITIONS
Components of the Climate System
Greatest variations in the composition of the atmosphere are associated with water (in its various phases). Other constituents also change (carbon dioxide, Ozone, for example), thus bringing into consideration atmospheric chemistry, ocean and land surface exchanges, etc.
The incoming solar radiation illuminates only part of the Earth, while the outgoing longwave radiation is distributed more evenly (Figure 1.2). On an annual basis, the result is an excess of absorbed solar radiation over the outgoing longwave radiation in the tropics, while there is a deficit at mid to high latitudes (far left), so that there is a requirement for pole-ward transport of heat in each of the two hemispheres (arrows) by the atmosphere and the oceans. The excess of net radiation at the tropics is about 68 Watts/m2 and the deficit peaks at about -100 Watts/m2 at the South pole and at about -125 Watts/m2 at the North Pole.
Radiative forcing, a change in Rn, can be classified as -
The amount of solar energy incident at the top of the atmosphere on an unit surface perpendicular to the beam of propagation is almost constant, (solar constant), and is about 1370 Watts/m2. However, on a horizontal surface, the amount of solar energy incident is about 342 Watts/m2, on an average during the year (Figure 1.3).
These radiatively active gases are called greenhouse gases because they act as a blanket for the thermal radiation emitted by the Earth's surface, causing it to be warmer than otherwise.
Clouds absorb and emit thermal radiation because they contain water in various phases, and thus they have a blanketing effect. However, clouds also reflect solar radiation. Therefore, clouds can act to both warm and cool the surface, depending on the type, structure, height, etc of the clouds. In our current climate, it has been determined that the net global effect of clouds is to cool the surface.
Mars is much smaller than the Earth, and its atmosphere is quite thin (about 1% of the Earth's). The Martian atmosphere consists principally of carbon dioxide, and there is small but significant greenhouse effect on Mars.
Venus is a much larger planet compared to the Earth, and its atmosphere is much thicker (about 100 times ours), and there is a very significant greenhouse effect, keeping the Venus surface at 500 degrees Celsius.
Anthropogenic Climate Change
Enhanced Greenhouse Effect
The Intergovernmental Panel on Climate Change (IPCC) concluded in 1995 that the increased amount of carbon dioxide is leading to climate change, and will produce, on an average, a global warming of the Earth's surface because of its enhanced greenhouse effect ...
If the concentration of CO2 in the atmosphere is double the preindustrial concentration, then, outgoing longwave radiation is reduced by about 4 Watts/m2, which can lead to a warming of the near surface by about 1.2 degrees Celsius.
But, because many other mechanisms come into play, and feedbacks, both positive and negative, kick into action, the best estimate for near surface warming is about 2.5 degrees Celsius, for a doubling of atmospheric CO2 concentration.
If all the CO2 in the atmosphere is removed, hypothetically speaking, then, outgoing longwave radiation is increased by about 30 Watts/m2, that is, about 7 to 8 times the 2xCO2 effect, with a corresponding cooling of the near surface. This is because, CO2 absorption is saturated in some parts of the spectral region where it tends to absorb; therefore longwave radiation absorption by CO2 tends to changes at a rate much slower than the rate of increase in the concentration of CO2.
A question, then is, what happens if one more than doubles the concentration of CO2 in the atmosphere? For each further doubling of atmospheric CO2 concentration, a further radiative forcing of about 4 Watts/m2 results, according to model projections. That is, concentration changes from 2xCO2 to 4xCO2 results in reduction of outgoing longwave radiation by about 4 Watts/m2. And, from 4xCO2 to 8xCO2 concentration change results in a further reduction of outgoing longwave radiation by another 4 Watts/m2. Similar changes in near surface temperatures may be expected.
As an excercise, can you make a plot of the changes in radiative forcing for successive doubling of atmospheric CO2 concentration?
Other Greenhouse Gases
Learn more about methane and other greenhouse gases
Aerosols also tend to act as nuclei around which water in clouds tends to condense. Hence, aerosols affect both absorption and reflection of shortwave radiation by clouds.
Aerosols may be injected into the atmosphere from natural sources. For example, dust blown off the deserts (dust from Sahara), or the eruption of volcanoes. However, human activities can also lead to increase in aerosol quantities in the atmosphere. As an example, most pollution from industry tends to inject sulphate aerosols into the atmospheres. Another source is deforestation or over-grazing, which denudes the ground, and can then result in dust from the top soil being air-borne.
Sulphate aerosols have been shown to cool the near-surface, principally by reflecting shortwave radiation. But, in no way, can this off-set the warming effect due to longer-lived greenhouse gases, and significant climate change can still result.
An example of positive feedback is the water vapor feedback in which the amount of water vapor in the atmosphere increases as the Earth warms and, because water vapor is an important greenhouse gas, it tends to amplify the warming.
Similarly, snow cover extent tends to decrease as the Earth warms due to the greenhouse effect. And, as the darker ground is exposed, more of incident solar radiation is absorbed, which leads to further warming of the near-surface, and a further decrease in the snow cover extent. This is called the snow cover-temperature feedback .
Clouds have both positive and negative feedback effects on warming. Clouds reflect solar radiation, thus cooling the surface (negative feedback), while they absorb longwave radiation, thus warming the surface (positive feedback). Clearly, which of the two dominates depends on the type of the cloud (structure, composition, cloud-top height, etc).
The ocean circulation is an effective means of distributing heat and fresh water around the globe. The oceans store heat, absorbed at the surface, for varying durations, and release it in different places, thus contributing to the variability of climate on many time scales.
Additionally, the thermohaline circulation (circulation driven by changes in water density due to temperature or salinity effects ) allows for water from the surface to carried into the deep abyss where it is isolated from atmospheric influences, and can thus store heat for a thousand or more years!
The oceans also absorb carbon dioxide and other gases, and exchange them with the atmosphere in ways that alter ocean circulation and climate variability.
The marine biota and ecosystems will also change with climate and can have important feedbacks on climate.
Vegetation on land plays a central role in the foodchain. Any changes in vegetation patterns of the world due to climate change or human activities are likely to be of serious consequences.
Consider deforestation. The burning of felled trees releases carbon dioxide into the atmosphere, and we know that carbon dioxide is a greenhouse gas. Exposing the soil results in soil particles becoming air-borne (aerosols). The reflectivity of the surface is now changed, and this bears on the heat and energy exchanges with the atmosphere, and ultimately, on the surface and near-surface air temperature.
Analysis of observations of surface temperature show that there has been a global mean warming of 0.3 to 0.6 degrees Celsius over the past 100 years.
The observed trend of a larger increase in minimum than maximum temperatures is apparently linked to associated increases in low-cloud amount, aerosol, and the enhanced greenhouse effect.
Changes in climate variability and extremes are begining to emerge, but global patterns are not yet apparent.
Recent evidence from ice core drilled through the Greenland ice sheet have indicated that changes in climate may often have been quite rapid and large, and not associated with any known external forcings.
Rates of change of radiative forcing induced by human activities are exceedingly rapid compared with the historical record. This raises questions about how, for instance, surface ecosystems might adapt to such rapid change.
Global climate models (GCMs) include as central components atmospheric and ocean general circulation models, as well as representation of land surface processes, sea-ice and all other processes shown in Figure 1.1
Models and their components are based upon physical principles represented by mathematical equations that describe the atmospheric and ocean dynamics and physics.
Such equations are solved numerically at a finite resolution using a three-dimensional grid over the globe. Typical resolutions used for simulations are about 250 km in the horizontal and 1 km in the vertical.
Because of such coarse spatial resolution, many of physical processes cannot be properly resolved, and one resorts to including their average effect through parametric representations (also called parameterization).
The coupling between the various components, especially between the atmosphere and ocean circulation models, has been challenging, and only recently and only in some cases, has this been done correctly.
A frontier and future research challenge is to bring more complete chemistry, biology and ecology into the climate system models to improve the representation of the various physical processes.
Reliable weather predictions can be made using atmospheric GCMs for periods of upto 10 days or so. For some parts of the world, however, reliable predictions of the climate can be made upto a year in advance; the example of El Niño Southern Oscillation (ENSO) comes to mind. However, all of this is internal climatic variability.
It appears that climate changes due to changes in external forcing can also be predicted, as is evidenced from several sources.
Predictability is a function of spatial scales. Atmospheric variability arising from internal instabilities is huge on small spatial scales. It is, however, the variability on large scales influenced by interactions of the atmosphere with other components of the climate system that is predictable.
Figure 1.4 shows the natural variability of the annual mean surface temperature on several different spatial scales from a climate model simulation for 200 years. This example highlights the much greater natural variability on small scales which makes detection of the small systematic signal, such as that might arise from enhanced greenhouse effect, much more difficult to achieve on regional scales.
The model is then run with changes in external forcing, such as with a possible future profile of greenhouse gas concentrations (also called experiments ). The difference between the climate statistics in the two simulations (control and experiment) provide an estimate of the accompanying climate change.
The long-term change in global annual mean surface temperature arising from a doubling of carbon dioxide concentration in the atmosphere is often used as a benchmark to compare models and to indicate the climatic sensitivity of the model (an example). For most models, this value is around 1.5o to 4.5o Celsius.
However, the concentrations of greenhouse gases will not level off at doubling, and the regional pattern of climate change depends significantly on the time-dependence of the external forcing. Thus, it is important to make future projections using different plausible evolving scenarios of anthropogenic forcing, so that the response of the climate to the forcing is properly simulated.