Question

I read recently that the planet Venus experienced a "runaway greenhouse effect" and that if this was to happen on Earth, the temperature would high enough for the oceans to boil. Is any of this "runaway warming in this century" stuff on the Internet plausible? And are the rumors true that the Earth has stopped warming and is now cooling (maybe headed for an ice age)?

The current global warming on Earth is extremely unlikely to lead to a runaway greenhouse effect, although to be honest scientists are not sure exactly what triggered the runaway greenhouse on Venus. But that does not mean that we can ignore the more modest warming that is happening now. What is a very small increase in global temperature of a few degrees could profoundly change our environment, for example by raising sea levels and flooding most the the largest cities on Earth. These changes do not threaten life on Earth, but they do threaten our economy and way of life. Climate changes are defined as those that take place over decades, not the shorter-term atmospheric effects that we call weather. The current global warming has been taking place for more than a century. Superimposed on the general warming are shorter-term changes, such as cooling after the injection of aerosols into the atmosphere by the eruption of Mt Pinatubo in 1991, and higher temperatures in the late 1990s associated with El Nino effects. The El Nino probably plays a part in the leveling of global temperatures in the past few years. The two hottest years on record are 1998 and 2005. While the directly measured temperatures have leveled, the large-scale effects of global warming are continuing, including melting of mountain glaciers and of the icecaps in Greenland and the Antarctic, and the dramatic shrinkage of the Arctic Sea ice cover.

The is a lot of misinformation and disinformation about global warming on the Internet, driven in part by political and economic issues. Following in italic are some “red flag” arguments from global warming deniers that can help you spot disinformation. (1) We should not worry about carbon dioxide, since the main greenhouse gas is water vapor. This is partly true, but it is the carbon dioxide (and methane) that control the structure of the atmosphere. Water vapor content is highly variable and essentially follows the carbon dioxide. (2) What we are seeing is “natural variations”, caused primarily variations in solar output. This is false; we have been monitoring solar energy for a quarter century, and the small variations are taken into account in all the climate models. (3) The apparent increase in temperature is an artifact, caused by the fact that much of the data is from cities, which are warmer than the surroundings. This is also wrong; the “heat island” effect has been corrected in plots of global temperatures. (4) While temperatures seem to have been rising in the lower atmosphere (the troposphere), they are dropping in the stratosphere. People who say this don’t realize this is the expected signature of greenhouse warming. If there were an external cause, such as increased energy from the Sun, both troposphere and stratosphere would be heating. (5) This is no big deal; temperatures are actually lower than they were in the Medieval warm period. This is wrong; over at least the past few thousand years, temperatures have never been as high as they are today. (6) More carbon dioxide is good, since it makes plants grow better. Yes, if we could increase carbon dioxide without greenhouse heating, this might be true, but high temperatures are not good for most plants, and the increase in carbon dioxide is also acidifying the oceans, which can be very bad, for example by destroying coral reefs. (7) There is no consensus; many scientists disagree about global warming. Hardly any peer-reviewed scientific papers have been published by dissenters in the past decade; the dissenter are mostly not climate scientists, and they have offered no alternative models to explain the data. The national academies of science in all of the industrialized countries have endorsed the findings of the Intergovernmental Panel on Climate Change, which represents a strong scientific consensus on both the reality of global warming and the challenges it implies.

David Morrison
NAI Senior Scientist
November 5, 2009