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The Uncertain Science of Climate Change

Do current climate change models offer an accurate view of global warming?

September 7, 2005

Do current climate change models offer an accurate view of global warming?

While there is more or less a general consensus about the danger of greenhouse gases and their role in global warming, the effectiveness of the science behind these predictions has not gotten much attention. This Globalist Document from the British House of Lords explores climate change and the models used to measure it.

The Earth’s surface is warmed by the sun. This incoming solar radiation is fairly constant — it does not vary with time.

The Earth's temperature is controlled by balancing between this incoming short-wave radiation, which warms the Earth, and the loss of this energy as it is bounced back into space.

The re-radiated energy cools the Earth. Energy-out balances energy-in — and the Earth maintains a constant global temperature. Without this balance, the Earth would become steadily hotter and life would cease.

Of the incoming solar radiation, roughly 30% bounces back into space from clouds, atmospheric aerosols and bright, reflective areas of the Earth's surface, such as deserts.

That leaves 70% of the incoming radiation to be absorbed, mostly by land areas and the oceans. But even this 70% cannot stay permanently absorbed, otherwise the Earth would again continually warm up and life would not be possible. This radiation is re-emitted primarily as long-wave, infrared radiation back into space.

But some of this re-radiated energy is absorbed by water vapor and by "greenhouse gases" that exist in the atmosphere. The principal greenhouse gas is carbon dioxide, but the principal absorbing agent is overwhelmingly water vapor.

The effect of this absorption of the re-radiated energy is to produce another round of re-radiation, this time back to the Earth's surface, where it is absorbed once again. This is the "greenhouse effect."

This re-absorption process is natural — it is what maintains the Earth's average temperature at + 15° Celsius (59° Fahrenheit), rather than at levels below freezing.

It is not this natural greenhouse effect that gives rise to concern. It is the fact that the relatively short period in the world's history since the Industrial Revolution has seen significant increases in the emissions of the greenhouse gases, especially carbon dioxide and methane.

These greenhouse gases add to the concentrations already in the atmosphere. Moreover, they accumulate and stay in the atmosphere for decades (their "atmospheric residence time").

While they get generally mixed in the atmosphere, it is common in pictorial terms to show these increased concentrations as a "blanket" that traps the outgoing long-wave radiation and returns it to Earth.

It is this accelerated or enhanced greenhouse effect that causes the concern, since the effect is to warm the Earth's surface even more than the level achieved naturally.

In effect, what is happening is that the greenhouse gases are upsetting the natural energy balance in such a way that “something has to give" to restore the balance — and it is surface warming that is bringing about the adjustment.

Evidence from Antarctic ice cores suggests that atmospheric concentrations of CO2 were fairly constant over 1,000 years until the Industrial Revolution. In the year 1000 (measured by ice core samples), concentrations were 280 ppm (parts per million). The concentrations were the same around 1800 — whereas today they are some 375 ppm.

Currently, concentrations are growing at some 1.5 ppm every year, as recorded by the Mauna Loa observatory in Hawaii, which has been monitoring concentrations since 1959.

Not all greenhouse gases — gases that contribute in some way to the enhanced greenhouse effect — create "positive forcing," that is, warm the atmosphere. Some have a cooling effect. Aerosols — tiny particles of liquid or dust in the atmosphere, such as soot, volcanic ash and dust — give rise to cooling effects.

Clouds can have a cooling effect as well, reflecting radiation back into space. The level of understanding of the behavior of clouds and aerosols is unfortunately far less than the level of understanding for the main warming greenhouse gases.

An important cooling aerosol is sulfate, which comes from sulfur dioxide (when mixed with oxygen), which in turn comes from sulfur-bearing fossil fuels, such as coal.

These sulfate aerosols reflect sunlight and hence produce a cooling or "dimming" effect. In the rich world, substantial controls exist over sulfur emissions because of damage caused by local air pollution and transboundary acid rain.

As a result, sulfur emissions are declining. But in the poorer world, there are still considerable pressures to burn fuels such as coal and lignite — and sulfur emissions are rising. The scenarios of future warming therefore depend in part on what happens to this balance of sulfur emissions.

Since greenhouse gas emissions — especially carbon dioxide — increased with the onset of the Industrial Revolution, current temperatures should have responded to these past emissions.

The recent historical record of global temperature change is not disputed. Instrumental records (using thermometers) for temperatures in the Earth's Northern Hemisphere do not really begin until around 1860.

"Global" (that is Northern Hemisphere) average temperatures show marked fluctuations around a rising trend. The approximate observed cycles are upwards for 1860 to 1875, downwards to 1890, up to 1900, down to 1915, up to 1942, down to 1970 — and upwards since then.

A more "smoothed" series would suggest a reasonably constant temperature to 1920, upwards to around 1940, downwards to 1970 and upwards since then. Since 1860, the mean temperature change has been around 0.6°Celsius (42.8° Fahrenheit).

Any test of the link between temperature change and greenhouse gases must therefore account for these cycles. Mathematical models that try to explain temperature change are known as general circulation models (GCMs). These models attempt to mimic the forces at work that change the Earth's climate.

If they can "explain the past," then they can be used to predict the future, assuming we have a reasonable idea of how the various determining factors — such as the greenhouse gases themselves — will behave in the future. GCMs tend to be very complex and have to run on powerful computers.

Testing the validity of climate models is obviously difficult. In so far as the models predict climate change, the predictions can easily be in error and only the passage of time can validate the predictions.

But if the science of climate change is correct, the option of "waiting and seeing" may be risky because of the manner in which current emissions of greenhouse gases add to the stock of gases in the atmosphere.

Adapted from a report published by the House of Lords, July 6, 2005. For the full-length report, click here.