The Greenhouse Effect
The greenhouse effect is the cause of global warming. The term refers to the way that certain gases in the atmosphere retain the heat which comes from the sun, instead of allowing the earth to re-radiate it back into space.
The effect is illustrated in the diagram on the right. Incoming solar radiation passes unimpeded through the atmosphere (except for UV, which is mostly stopped by the ozone layer), and warms the earth. The warmed earth then emits heat -- infra-red electromagnetic radiation. Crucially, this radiation has a different wavelength from the incoming solar radiation, and a much larger proportion is trapped by 'greenhouse gases' in the atmosphere -- carbon dioxide (CO2), methane (CH4) and ordinary water vapour (H20). The more of these gases there are in the atmosphere, the more heat is trapped, and the more the earth's surface warms up.
The greenhouse effect is not all bad. If there were no greenhouse gases at all then the surface of the earth would be at roughly -18 degrees Centigrade, too cold for any life to exist. But increasing the level of carbon dioxide and methane, as we are doing by burning fossil fuels, is raising the earth's surface temperature.
The red line on the graph shows the level of CO2 in the atmosphere, measured in Hawaii, in the middle of the Pacific Ocean. It fluctuates each year, decreasing between March and September as plants grow new leaves in the northern hemisphere, where there is more vegetation than in the south, and rising again after northern autumn leaf fall. The overall rise is due to our burning fossil fuels -- coal, oil and gas.
At the current rate of increase, we will have doubled the pre-industrial level of atmospheric CO2 by 2060. The climate sensitivity is the temperature increase this doubling would lead to. It is hard to estimate how much this will be, because of the complexity of atmospheric processes and the many feedbacks involved -- for example, the melting of snow and ice darkens the polar and high mountain regions, leading to more rapid warming. Other feedbacks may speed up warming or slow it down. Overall, the climate would take a very long time -- perhaps even millennia - to reach a new equilibrium, even if CO2 levels stabilised.
The sixth IPCC report estimates the most likely value of the Equilibrium Climate Sensitivity at 3 degrees C. The Transient Climate Response to CO2 doubling is likely to be about 2 degrees C.
Extreme Events and Feedbacks
A temperature increase of about 2 degrees Centigrade
doesn't sound like anything to worry about, but there are
several reasons why it would be a problem.
- Increased energy in the atmosphere gives rise to more extreme events-- heatwaves, storms and floods. A single event can be an insurmountable barrier to the survival of a town, a species, or a way of life.
- "2 degrees" is only an average, and in some places the rise is much greater. In particular, any rise reduces the amount of snow and ice at the poles. As the snow retreats, revealing the darker land or water beneath, more of the sun's heat is absorbed, and local temperatures rise rapidly. In the spring of 2022, polar temperatures were up to 40 degrees C above normal. The resulting melting of the ice-caps will eventually raise global sea levels by about 60 metres. This may take centuries, but polar warming is now happening four times faster than the world average, exceeding even alarmist predictions.
More Climate Science
A lot of information on climate science is now available online, with varying levels of reliability. The climate change pages of NASA and the United Nations reflect a mainstream scientific consensus. Both give a more worrying picture than you might expect. The website run by Skeptical Science helps to rebut some of the misinformation that also circulates. Recently, the new field of Attribution Science has developed ways of estimating the role of climate change in individual events, using climate modelling and computer simulations. Attribution Science will play a crucial role in legal cases where plaintiffs sue energy companies for the damage caused by climate change.