A summary of the United Nations Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC Report) was released on February 2, and many in the media and Congress are citing it as further evidence that global warming is a dire threat. The full report, with accompanying scientific assessment and detailed assumptions, will not be released for several months. However, caution is warranted in drawing policy conclusions based on this summary, as the full scientific debate over the IPCC report has not begun. And while the summary strongly emphasizes mankind's role in global warming, it has retreated on a number of important assertions from past reports.

Just a Summary

It should be emphasized that only a short "Summary for Policymakers" has been released, not the actual report which contains the underlying scientific assessment. The final version of the full report is scheduled to come out later this year. IPCC summaries are written at the direction of political appointees representing member nations. The limitations and potential biases of such summaries give reason to withhold judgment until the scientists actually weigh in--both the IPCC scientists and especially the independent scientists who will comment on the final report. That the summary is being so aggressively marketed ahead of the science is itself reason for caution.

The Findings

That said, the summary is the only thing most journalists and politicians read, and the finding that has received the most attention is that the IPCC is now more certain than in its 2001 report that mankind has contributed to global warming since 1750. In truth, few so-called skeptics dispute that there has been some human contribution, so the fact that the summary says the likelihood is 90 percent or more is not as newsworthy as it first appears. This upward revision in the certainty that mankind has impacted the climate should not be confused with an upward revision in the predictions of consequent harm.

The more important questions have always been the extent of warming, the seriousness of the consequences, and what responsive policies make sense.

The summary includes a wide range of assumptions and outcomes, and thus it is hard to generalize about its predictions. However, it does appear that estimates of future sea level rise--likely the greatest concern from warming--are being revised downward. Estimates range from 0.18 to 0.59 meters (about 7 to 23 inches) over the course of a century, about a third lower than in the previous report and well below popular fears of 20 feet or more.

Again it is still too early to speculate what the final scientific assessment will say (and how well it will hold up to scrutiny), but the summary does appear to have backtracked on other points as well. For example, the last IPCC report emphasized the so-called "hockey stick" notion that earth's temperature was relatively stable for a thousand years (the shaft of the hockey stick) and then shot up in an unprecedented manner in the 20th century (the blade). Thus, the previous IPCC report discounted the Medieval Warm Period and subsequent Little Ice Age, implying that current temperature increases are not due to natural variability. The hockey stick (and its conclusion that current temperatures are unprecedented throughout most of recorded history) has come under scientific attack in recent years, and based on the new summary, it appears that the IPCC has deemphasized it. How the deletion of the hockey stick from the upcoming report squares with IPCC's claims of increased certainty over mankind's impact on climate will be a significant source of contention.

On the question of whether global warming contributes to powerful hurricanes like Katrina, the summary hedges quite a bit, calling the hurricane-warming link "more likely than not" rather than "very likely" or "likely," as used elsewhere in the summary. The summary concedes in a footnote that the magnitude of mankind's contribution was not assessed and that the attribution was based "on expert judgment" and not formal studies. Again, depending on what the final report says, activists and politicians who unequivocally blamed Katrina's devastation on global warming may have to back off.

Is a Kyoto-Like Solution Wise?

Whatever the risks of global warming identified in the IPCC Report, global warming policies also carry risks--especially those policies that emphasize energy rationing as a solution. Separate from the scientific discussion sparked by the IPCC Report is the discussion about the appropriate response and, in particular, the actual merits of a costly program to cap carbon dioxide emissions from fossil fuel use. This was the approach taken in the Kyoto Protocol, the multilateral treaty to address global warming. Kyoto is proving so prohibitively expensive that most of the developed nations that have signed onto it (Western Europe, Canada, Japan, but not the United States) have little hope of meeting its looming targets. The economic burdens of Kyoto, especially on already-stagnant economies, have proven unacceptable.

Estimates of the cost for U.S. compliance ranged from $100 to $400 billion dollars annually--enough to have a serious impact on employment and economic growth. America wisely rejected this approach. And even assuming the IPCC is right, the Kyoto Protocol would only avert 0.07 degrees Celsius of warming by 2050, an amount too small too measure.

The economic damage of energy rationing to developing nations would be severe, strangling growth and imposing hardships on billions who are already barely subsisting. It would also slow the spread of electrification to the nearly 2 billion who do not yet have it. On the other hand, exempting developing nations from any Kyoto-style requirements means that carbon emissions will continue to increase regardless of what the developed world does.

Science should play a big role in global warming policy, and the full IPCC Report should be a part of that. But economics must also play a role, lest the U.S. embark on a course that does more harm than good.

Ben Lieberman is Senior Policy Analyst in the Thomas A. Roe Institute for Economic Policy Studies at The Heritage Foundation.