HadCRUT3 yearly global surface temperature variations, 1900 through 2011

What is global warming?

In its most commonly used sense, “global warming” refers to the gradual warming of global-average temperatures due to the slowly increasing concentrations of man-made atmospheric greenhouse gases, primarily carbon dioxide. But global warming can alternatively refer to simply the observation of warming, without iplying the cause(s) of that warming.

As seen in the above chart, there have been two main episodes of warming in the last 100 years or so...before 1940, and then from the 1970s to about 2000. Warming stopped around 2000.

The most popular explanation for global warming is the burning of fossil fuels, mainly petroleum and coal, which produces carbon dioxide as one of the by-products. As of 2010, the concentration of carbon dioxide is about 50% higher than it was before the start of the industrial revolution in the late 1800's. The potential warming effect of the extra CO2 is through its ability to absorb and emit infrared radiation, which is the type of radiation the Earth continually loses to outer space in response to heating by sunlight. This makes carbon dioxide a greenhouse gas, albeit a weaker one in the atmosphere than water vapor.

The net effect of greehouse gases on is to keep the lower layers of the atmosphere warmer that they otherwise would be without those gases. Therefore, it has seemed reasonable to assume that an increase in greenhouse gases would lead to warming.

But the big question is, how much warming?

How serious will global warming be? We don't know for sure, and there is much debate in the climate research community over this very question. The consensus of opinion is that a warming of about 0.2 to 0.3 degrees Celsius (about 0.4 to 0.6 deg. F) every 10 years is expected for the next 100 years or so.

The reason why there is so much uncertainty, though, is not because of the direct warming effects of more carbon dioixde, which are relatively weak. The main uncertainty is whether clouds and other components of the climate system will change in a way that amplifies -- or reduces -- the relatively weak warming effect of the extra carbon dioxide. These indirect effects are called feedbacks, and they will determine the extent to which manmade global warming is a problem or not.

The processes by which clouds and water vapor -- the 2 main feedbacks -- are maintained in the atmosphere are very complex. For instance, clouds (on a whole) act to cool the Earth in response to heating by the sun. But the consensus of scientific opinion is that this will reverse with the slight warming from increasing carbon dioxide. Again, how clouds will change with global warming will be critical, as they could either amplify the warming, or reduce it.

While water vapor feedback is more widely accepted to be an amplifying influence on warming, this is not obvious since warming-induced change in precipitation formation processes in rain systems is not well understood, and could cancel out the greater surface evaporation rates that would accompany surface warming. If those rain systems become more efficient at converting water vapor into precipitation, this would act to offset global warming.

Cloud and precipitation-related feedbacks are so uncertain that it is difficult to assign any level of confidence to global warming projections.

But what about the warming to date? While there is some uncertainty over how much warming has occurred over the last 50 to 100 years, it is pretty certain that warming has, indeed, occurred.

But is that warming unusual? The following plot shows an average of 18 different temperature proxies over the last 2,000 years (not including tree rings, which many believe to be unreliable indicators of temperature):
2,000 years of global temperature proxies, from Craig Loehle
As can be seen, every century shows evidence of global warming or global cooling. What could cause such changes? No one knows for sure, although there are a few ideas, mostly related to natural variations in global cloud cover. These changes in Earth's natural "sun shade" need be only 1% or 2% to cause global warming -- or cooling.

In fact, it is entirely possible that warming in the last 100 years has been more natural than manmade. This would require that net feedbacks be a warming-reducing influence.

This would also mean that future manmade warming will likely be much weaker than currently assumed by the "scientific consensus".

Finally, the potential role of the oceans in global temperature change is not well understood. Most of the oceans below a few hundred feet depth are very cold, and so a change in the overturning circulation of the ocean can cause warming or cooling. Because the ocean (and the atmosphere) is a nonlinear dynamical system both are subject to chaotic changes without any external forcing. Science would have difficulty predicting any such chaotic changes, if they exist.

(page last updated 6/1/2012)
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