Chapter 1: Overview of the Climate System

1.1. THE CLIMATE SYSTEM AND SOME DEFINITIONS
1.2. RADIATION BUDGET
1.3. THE GREENHOUSE EFFECT
1.4. CLIMATIC RESPONSES
1.5. OBSERVED CLIMATE CHANGE
1.6. PREDICTION AND MODELLING OF CLIMATE CHANGE
FURTHER READINGS

1.1. The Climate System and Some Definitions

Weather
The large fluctuations in the atmosphere from hour-to-hour or day-to-day constitute the weather. The weather systems arise mainly due to atmospheric instabilities, and their evolution is governed by non-linear chaotic dynamics. Which is why, weather is not really predictable beyond a week or two into the future.

Learn more about chaos

Climate
Climate is defined as averaged weather, typically defined in terms of mean and other statistical quantities (higher order moments), that measure variability over a period of time and over a geographical region (space).

Climatic Variations
Climate involves variations (that is, changes) of two kinds -

Learn more about Climate Variability
Learn more ENSO

Components of the Climate System
The internal components of the climate system include (Figure 1.1)

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.

Solar Constant
At the top of the atmosphere, the amount of solar energy incident on a unit surface area, perpendicular to the direction of propagation, is called the Solar Constant. It is about 1370 Watts/m²

Radiative Balance
At the top of the atmosphere, the expenditure all incident solar radiation must be accounted for the Earth as a whole, that is, the net incoming solar radiation must be balanced by the outgoing longwave radiation. (background info!)

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.

Learn more about electromagnetic radiation
Learn more about Radiation Laws
Learn about Atmosphere Effects on Earth's and Solar Radiation

Radiative Forcing
Net radiation at the top of the atmosphere (Rn) is defined as the difference between net incident solar radiation (Rs) and the outgoing longwave radiation (Rl). That is, Rn = Rs - Rl, for the Earth as a whole, on an annual scale. A change in Rn is defined as Radiative Forcing of the Climate.

Radiative forcing, a change in Rn, can be classified as -

Climate System
The totality of the atmosphere, hydrosphere, biosphere, geosphere and their interactions is defined as the Climate System.

Climate Change
A change in climate which is attributed directly or indirectly to human activity that alters the composition of the atmosphere and which is in addition to natural climate variability observed over comparable time periods. A much better definition of climate change, is simply, climatic variations due to human activities.

What is Climate Change?

Climate Variations
Variations due to natural changes in the system. Thus, a key goal of climate research is to separate and identify variations of the climate system due to human activities from natural variability of the system. Note, however, attribution is another prickly issue.

Basic Math Refresher

1.2. Radiation Budget

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).

The Earth's Radiation Energy Balance
The Radiation Budget of the Earth

1.3. The Greenhouse Effect

Greenhouse Effect
Most of the atmosphere consists of Nitrogen and Oxygen (99%), and is transparent to the longwave radiation. Water vapor (about 2%) and other gases (in minute concentrations; hence called trace gases ) such as carbon dioxide, methane, etc,absorb longwave radiation emitted by the Earth's surface, and generally re-emit from much higher and colder levels to the outer space.

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.

The Greenhouse Effect
More About the Greenhouse Effect

Anthropogenic Climate Change
Human activities can lead to changes in the composition of the atmosphere, which in turn can lead to radiative forcing of the climate system, thus resulting in climate change. For instance, burning of fossil fuels which increases atmospheric carbon dioxide concentration, deforestation which changes the Earth-Atmosphere radiative exchange, and pollution which increases the aerosol concentration, can in principle result in radiative forcing of the climate system.

Learn more about biomass burning and production of greenhouse gases

Enhanced Greenhouse Effect
The concentration of carbon dioxide (CO2) in the atmosphere has increased by about 25% in the past 100 years, mostly due to human activities, such as combustion of fossil fuels and deforestation . Its concentration at the present time is about 370 parts per million by volume (ppmv). The projections are that the concentration of CO2 will be double the pre-industrial concentration (270 ppmv) within the next 50 to 100 years, in the absence of any controls.

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?

Learn about Greenhouse Gases

Other Greenhouse Gases
There are important greenhouse gases, other than carbon dioxide, such as methane, Nitrous Oxide, and Tropospheric Ozone. Changes in the concentration of these gases has been documented, and attributed to human activities; for example, biomass burning, landfills, rice paddies, animal husbandry, industrial activities, and so on. The greenhouse effect due to these gases re-enforces the CO2 greenhouse effect.

Learn more about methane and other greenhouse gases

Aerosol Effects
Most aerosols in the atmosphere reflect solar radiation back to the outer space. Therefore, the aerosol effect is to principally cool the surface.

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.

Aerosols and Climate Change
Atmospheric aerosols and climate

1.4. Climatic Responses

Feedbacks
The increase in greenhouse gases and aerosol concentration results in a radiative forcing of the climate system. The determination of the climatic response to this forcing is complicated by the fact that feedbacks come into play. Some of these can amplify the original warming (positive feedbacks), while others can dampen it (negative feedbacks).

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).

Learn about feedbacks

Oceans
The oceans cover about 70% of the Earth's surface, and through their fluid motions, high heat capacity, and ecosystems, they play a most important role in shaping the Earth's climate and its variability.

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.

Ocean Currents and Climate Change

Land
The penetration of heat into the land associated with the annual cycle of surface temperature is only about 2 meters, and the heat capacity of land is much smaller than water of comparable depth. One result is, near-surface air temperature changes over land happen much faster and are much larger than over oceans, for the same amount of heating. And, since we live on land, this directly affects human activities.

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.

Learn more about past vegetation changes

1.5. Observed Climate Change

Observations
Most observations have been made for purposes other than detecting climate change, such as, weather forecasting. These observations suffer from changes in instrumentation, exposure, measurement techniques, station location, observation times, etc. Thus, the high quality of much needed long-time series of observations is often compromised and therefore, special care is required in their interpretation. For the more distant past, proxy data from climate-sensitive phenomena, such as from tree-rings, ice cores, and pollen in marine sediments are used.

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.

Learn about trends in U.S.climate during the 20th century

1.6. Prediction and Modelling of Climate Change

Climate Models
In order to quantify the response of the climate system to changes in forcing, one must account for all the complex interactions and feedbacks between the various components of the climate system. As this is not possible to do reliably using statistical methods, one resorts to numerical models of the climate system based on sound well-established physical principles.

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.

Learn more about climate models

Climate Predictability
To what extent is climate predictable? That is, are there any signals large enough to be distinguished from the noise of natural variability that may be potentially predictable.

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.

Climate Projections
When a model is employed for climate prediction, it is first run for many simulated decades without any changes in external forcing of the system (also called control run ). The quality of the simulation can then be assessed by comparing the mean, the annual cycle and the variability statistics on different time scales with observations of the climate. In this fashion, a climate model may be evaluated .

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 1.5^o to 4.5^o 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.

Climate Impact of Quadrupling Atmospheric CO2

Further Readings
IPCC 2001: Chapter 01
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Aug-30-2002
rmyneni@bu.edu