Global warming, due to greenhouse gas accumulation in the atmosphere, is causing rapid climate change. We can slow global warming by reducing greenhouse gas emissions.
How fast is the world warming?
Global average near-surface atmospheric temperature rose by around 0.7 degrees during the 20th century (0.07 degrees per decade), and by around 0.5 degrees C between 1970 and 2000 (0.17 degrees per decade).
For more detailed temperature graphs, go to NOAA - Global Climate at a Glance.
Can we slow down global warming?
A significant portion of global warming over the last century is due to accumulating greenhouse gases, which means that we can slow down global warming by reducing greenhouse emissions, and hence slowing greenhouse gas accumulation.
How much should we slow down global warming?
Global warming is occuring at a rapid rate - 0.18 degrees C per decade between 1976 and 2006 (WMO). If we were to slow global warming to below 0.1 degrees C per decade, this would give both humans and nature more time to adapt.
What's causing global warming?
Rising concentrations of greenhouse gases in the earth's atmosphere are making it less transparent to outgoing infrared radiation. This has resulted in more energy coming in from the sun than is radiated back out into space. Hence global warming.
Greenhouse gas levels (plus other factors affecting how much sunlight gets in, and how much infrared gets out) determine the earth's "equilibrium temperature" - how hot the earth (surface and atmosphere) must glow in order to give off sufficient infrared energy for enough if it to get out past the atmosphere to balance the energy coming in from the sun.
Greenhouse gas levels having risen, and continuing to rise, the earth will continue to get hotter until it reaches the equilibrium temperature corresponding to the greenhouse levels.
How hot will it get?
Stabilising greenhouse gas concentrations will stabilise the earth's equilibrium temperature, thus setting a limit on how hot the earth will get before the rate of global warming slows down. Just how long it will take before equilibrium temperature is reached depends on how much heat is being drawn away from the earth's surface by its oceans and other heat sinks.
In the meanwhile, other factors may affect the equilibrium temperature. For example, global warming may increase the extent of the earth's cloud cover, thus reducing the amount of energy coming in from the sun, and so lowering the equlibrium temperature. On the other hand, it may increase the amount of water vapor in the atmosphere, thus raising the equilibrium temperature. (Atmospheric water vapor also blocks outgoing infrared radiation.)
Our "thermal equilibrium" model of global warming suggests that the rate of global warming is determined by the gap between the equilibrium temperature - as reset by changing greenhouse gas concentrations - and the earth's temperature at a given time.
As the difference between ingoing and outgoing energy flows increase (due to changing greenhouse levels), net energy flow - and hence the rate of global warming or cooling - also increases. As the difference returns to zero (as equibrium is approached), net energy flow decreases.
Thus, if we reduce greenhouse emission rates, hence slow greenhouse accumulation, hence slow the gradual rise in the equilibrium temperature, we may allow the actual temperature of the earth to catch up. As the gap between actual temperature and equilibrium temperature decreases, then (hopfully) global warming will slow down.
What's wrong with global warming?
Global warming is causing climate change (change in average local temperatures and rainfall), greater weather extremes, and rising sea levels. This will add to the disruption of natural habitats by land clearing that has already occured, and in many areas still occuring.
The increasing incidence and severity of floods, storms and droughts will disproportionately affect the world's poorer inhabitants, many of whom are not sharing in the benefits of the economic activities responsible for global warming.
But perhaps the main problem with global warming is that it is happening too quickly. In general, survival and prosperity comes through adapting to the conditions we find ourselves in. But if conditions change too quickly, adaption becomes very difficult. We can live with climate change, but not if it is too abrupt.
How to respond to global warming
We can respond to global warming by both adapting to its effects and by trying to slow it down.
We can adapt to global warming by, for example, changing agricultural practices in regions affected by drought, strengthening city defences against flooding and storms, and moving populations to less affected areas, if necessary. And we can offset climatic disruption to natural ecosystems, by, for example, decreasing the amount of land devoted to agriculture in order to expand the area of land set aside for nature conservation parks, etc.
We can slow global warming down by shifting from a high-energy-intensive economy dependent on fossil fuels, to a low-energy-intensive economy able to get by on renewable energy.
Why not both adapt to global warming and try to slow it?
Some people say that we shouldn't try to stop global warming, but concentrate on funding adaptation measures instead.
They argue that, in order to afford adaptation measures on a significant scale, we need continued economic growth, and the only way to ensure continued economic growth is to continue consuming fossil fuels at the same or greater rates than now.
While increased fossil fuel combustion may make global warming worse, the payoff from the economic prosperity thus gained would amply compensate for the increased costs that this additional warming would entail, they say.
Problems with this argument:
(1) Measures to slow global warming are likely to be cheaper than the cost of adapting to the consequences that would otherwise occur. (Refer Stern Report).
(2) The extra wealth (if any) from unabated fossil fuel consumption may end up not being channeled into the adaptation measures needed to compensate for the problems so caused. (Some still live by the motto: keep the benefits, redistribute the costs.)
(3) It is not obvious that continued high rates of fossil fuel consumption are necessary to economic growth, or at least to being able to fund large scale adaptation measures while also maintaining and extending economic properity.
How to slow global warming
Global warming can be slowed by stopping, and if necessary reversing the accumulation of greenhouse gases in the atmosphere. In particular, by stabilising or reducing atmospheric concentrations of carbon dioxide, methane and nitrous oxide (the main greenhouse gases).
How to stabilise atmospheric greenhouse gases.
Stopping accumulation will require reducing greenhouse emission rates until they are in balance with the rate at which such gases are naturally removed from the atmosphere. (The latter occurs through uptake by the world's oceans, forests and other carbon sinks or spontaneous molecular decay).
How quickly do we have to stop greenhouse gas accumulation in the atmosphere?
Natural mechanisms may respond to global warming or greenhouse gas accumulation by increasing uptake of these gases. For example, vegetation may respond to warmer conditions by absorpting more carbon dioxide. On the other hand, other mechanisms may respond reducing uptake or releasing stored greenhouse gases. For example, siberian peat bogs may release large amounts of carbon dioxide, or minerals on the seabed may release large amount of methane.
Natural mechanicms such as increasing cloud cover may repond to global warming by reducing the amount of heat reacching the earth from the sun, in which case we may not need to stabilise greenhouse concentrations so quickly. But such mechanisms may respond very slowly, or may be counteracted by other natural mechanisms that respond to global warming by accelerating it.
In the absence of more definite knowledge about how the climate system will respond to global warming, we should apply the one method we believe does have a good chance of slowing it, namely reversing the accumulation of greenhouse gases in the atmosphere as quickly as possible.
Current Warming Rate
How fast is the world warming?
World Meterological Organisation
WMO Statement on the Status of the global Climate in 2006 (DOC). World Meteorological Organization, 2006.12.14
The global mean surface temperature in 2006 is currently estimated to be +0.42°C above the 1961-1990 annual average (14°C).
Since the start of the 20th century, the global average surface temperature has risen approximately 0.7°C. But this rise has not been continuous.
Since 1976, the global average temperature has risen sharply, at 0.18°C per decade. In the northern and southern hemispheres, the period 1997-2006 averaged 0.53°C and 0.27°C above the 1961-1990 mean, respectively.
International Panel on Climate Change
Climate Change 2007: The Physical Science Basis - Summary for Policymakers. (PDF) International Panel on Climate Change, 2007.2
The linear warming trend over the last 50 years, 0.13 (0.10 to 0.16)°C per decade, is nearly twice that for the last 100 years. The total temperature increase from 1850–1899 to 2001–2005 is 0.76 (0.57 to 0.95)°C.
Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.
Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.
The Atmosphere Effect, University of Wisconsin
The Causes of Global Climate Change, Pew Centre on Global Climate Change
Efficacy of Climate Forcings. Hansen et al., GISS, 2005
Global Temperature Change (PDF). Hansen et al., PNAS, 2006.9
Global Warming, Answers.com
Global Warming, University of Oregon
Greenhouse Effect, University of Oregon