This is part of a much longer commentary in the NY Times on the evidence for global warming:
The Evidence for Global Warming, by Philip M. Boffey, Commentary, NY Times: While the debate over what to do about global warming heats up ..., scientists have made substantial progress in recent years in defining the threat and estimating its likely impacts. The picture they paint is worrisome. The evidence suggests that humans are altering the atmosphere in ways never before seen. The only question is how damaging the consequences might be, and what can be done to head off or adapt to the worst...
Skeptics say these things are most likely part of the natural variation of Earth's climate, unrelated to man-made warming. ... [G]iven the huge potential consequence of the debate, it's important to examine all the evidence carefully. So let's look at the various pieces of the global warming debate one at a time.
The biggest question is the one on which there is least dispute. The leading scientific organizations with relevant expertise have overwhelmingly adopted the view that human-induced global warming is a serious problem. ... Only the American Association of Petroleum Geologists, with deep ties to the fossil fuel industry, has demurred.
Meanwhile, the vast majority of research reports in leading scientific journals tend to support the prevailing view that human activities are mostly responsible for driving up temperatures. An analysis of 928 abstracts from leading scientific journals between 1993 and 2003 found that ... [n]ot a single paper disagreed with the consensus. ...
Still, there is plenty of disagreement over how fast the climate will change and how dire the consequences might be...
Analyses of the gases trapped in ancient ice cores from Antarctica have revealed that important greenhouse gases have reached their highest atmospheric concentrations in at least 650,000 years. The concentrations will only get worse... Other things being equal, the rise in these gases will cause temperatures to rise. That's simple physics, agreed to by all sides.
What's not agreed to is how worrisome the temperature increase will be. The global average surface temperature rose about 1 degree Fahrenheit over the 20th century. The change hardly seemed noticeable, except in polar regions where the increases were larger. Yet even that seemingly small increase is affecting the global environment by thawing the frozen tundra, melting mountain glaciers, adding to stress on coral reefs, causing some species to change habitats, and increasing the number of hot days while decreasing the number of cold days, to cite a few examples. And the warming trend may be picking up speed. The last few decades of the 20th century were probably the warmest in a thousand years.
Skeptics have an answer for this. They say surface temperatures were probably as high or higher during the Medieval Warm Period that ushered in the last millennium, well before humans emitted vast amounts of greenhouse gases. That suggests to them that today's warming might simply be a continuation of long-term natural cycles. But the magnitude and geographic extent of the warmth back then is uncertain. ...
And for the rest of this century, temperatures will almost certainly keep rising. The Earth has been storing heat in its oceans, which means there is about 1 degree Fahrenheit more warming ... that will occur during this century even without any additional greenhouse emissions. All major components of the climate system are warming — the lower atmosphere, the surface, and the seas — so the heating cannot readily be attributed to natural mechanisms that transfer heat from one part of the globe to another.
The projections for the future also get far more worrisome than that 1 degree. Various scenarios used by climate modelers suggest that average surface temperatures could easily rise another 4 to 8 degrees Fahrenheit by the end of the century, based on mid-range projections. That is a level that many experts deem dangerous.
If the warmer climate increases the destructive power of hurricanes and typhoons, as two studies indicate it already has, the storm devastation could get worse... If the massive ice sheets on Greenland and Antarctica melt faster than long estimated — a trend that some recent studies suggest has already started — the added water could drive up sea levels by several feet in this century, inundating some low-lying coastal areas. If mountain glaciers around the world continue to shrink rapidly, ... areas that rely on them to store water and release it slowly may face shortages of drinking water. If high temperatures allow disease-carrying insects and plant pests to invade new areas, as some studies show is beginning to happen, or if higher temperatures increase the frequency of heat waves and heavy rainfall, as the world's science academies deem likely, then the health and environmental consequences could be significant.
None of this is settled science or sure to happen. But these and other potential risks show what's at stake in the climate debate, and underscore the need to act promptly to head off the worst dangers. ... [big cut]
With all of the most prestigious scientific organizations convinced that global warming is an increasing menace — and with the vast majority of research articles in leading scientific journals tending to support that consensus — it would seem wildly irresponsible not to believe it is important to curb emissions. These are the institutions with the most expertise, and they have been studying the issue in unparalleled depth and breadth. Their judgment deserves the utmost respect and attention. ...
The world keeps pumping greenhouse gases into the atmosphere in what amounts to a huge uncontrolled experiment, and a gamble that all will turn out fine. But ... if the worst-case scenarios turn out to be accurate, we could be dooming much of the planet to a very unpleasant future.
To answer the global warming question, scientists have to separate the cyclical part of temperature variation from the trend, and then understand the sensitivity of the trend (and cycle) to changes in greenhouse gases.
Economists face a similar problem. An important debate in economics is how much of the variation in GDP is caused by supply shocks, and how much is caused by demand shocks. To answer this and other important questions, the cyclical part of GDP must be separated from the trend component. (Demand shocks have short-run, or cyclical effects, but do not affect the long-run trend; supply shocks can have both short-run and long-run effects. Thus, the trend is dependent upon supply side factors while the cycles can be affected by both demand and supply shocks. The cyclical variation is generally thought to be dominated by demand shocks, though that is not universally accepted).
In order to differentiate a change in the trend for GDP or other macroeconomic variables from a change in GDP around the trend, long time-series are needed, and the longer the better. For example, are recent increases in GDP growth driven by high levels of productivity part of a cycle where growth and productivity will return to lower historical levels with time, or is this a change in trend so that we can expect permanently higher productivity and growth? The answer is important for all sorts of questions such as how high tax collections - and hence the deficit - will be in the future.
Unfortunately, we do not have the equivalent of samples from ice cores from the distant past to guide us -- reliable economic data don't exist prior to around sixty years ago, and we are often limited to forty or so years of data (since 1959 since money data don't exist before then). Because of this, our ability to differentiate between the trend and cyclical components of economic variables is not as precise as we would like. New theory could help, but a longer span of data would be even better.