9 October 2005Kurt M. Cuffey
Mounting evidence has forced an end to any serious scientific debate on whether humans are causing global warming. This is an event of historical significance, but one obscured from public view by the arcane technical literature and the noise generated by perpetual partisans.
Fortunately, public attention is focused on the environment and the possibilities of climate change after this summer's devastating hurricanes and reports of disappearing Arctic Ocean ice.
It was only 15 years ago that public discussion of global warming became both prominent and strongly polarized. One group decided right away that scientists must be right and their most dire predictions taken seriously. The other group decided that most scientists were wrong, speaking from weak foundations and politically motivated.
But most people quietly adopted a more thoughtful position in the middle, recognizing the power of scientific claims while also adopting a healthy skepticism for claims that are new and dramatic.
In the scientific community, global warming claims were widely recognized as plausible and supported by evidence.
But several contrary arguments were sufficiently important to demand attention and soften warnings about the need for intervention. These included evidence contradicting observed warming and also theoretical concerns about climate models.
These contradictory arguments caused scientists such as myself to make fuzzy statements like "I am 80 percent sure that global warming is a problem," a phrase that I remember using while hoping no one would ask me to justify the exact choice of 80 percent.
These contrary arguments also fueled the fires of some passionate, well-funded and articulate contrarians, a small group of reputable scientists who chose to fight the dominant current. This was very healthy for the science, I think, but pronouncements from this group had an unfortunately large role in the polarized public discourse.
But now, after this summer of 2005, the serious scientific debate about global warming has ended. There is now no reasonable doubt that atmospheric pollution is causing global warming, and this warming is strong enough to have serious consequences in the next century.
Here are some of the recent results that buried the contrary arguments and showed climate models to be basically sound. All are based on extensive analyses accessible in the technical literature:
-- Glacier shrinkage in mountain ranges around the globe confirms the temperature trends shown by weather data (published April 2005).
-- Satellite and balloon measurements show the lower atmosphere is warming, commensurate with warming of the surface (published September 2005)
-- Measurements document the warming of the upper layers of the ocean, with a magnitude and patterning as expected from climate models. This means the Earth is absorbing more heat from the sun than it is releasing into space. The magnitude is precisely as expected from causation by atmospheric pollution (published June and July 2005).
-- Reconstructions of past climates, on timescales of millennia to millions of years, demonstrate that small changes in climate influences (like greenhouse gases) cause significant climate changes. And the magnitude of the changes is in the mid- to high range of predictions from the best climate models (published October 2004).
The evidence has forced me to drop the weaseling "80 percent sure" caveat. The evidence should compel the thoughtful skeptics -- from the offices of Chevron to the halls of Congress -- to accept that global warming is for real.
To use the legendary 1970s film, "Jaws," as metaphor: For the past 15 years, we have been listening to the scary music. Ladies and gentlemen, the shark's fin has just risen above the water! It is time for a broader and more serious conversation.
Why should we care?
The possible damaging effects are much discussed: rise in sea level, intensified drought, spread of diseases, extreme heat waves and intensified storms. The science here is less advanced and more speculative than one would like, but actively developing and solid enough for deep concern.
Consider the case of sea-level rise, caused by melting of the polar ice sheets and by thermal expansion of warming ocean waters. Sea-level rise will act with murderous consequences against coastal populations, not by itself, but in combination with major storms. Hurricane Katrina was not caused by global warming, but the New Orleans disaster violently illustrates the dangers at the conjunction of storm, sea and low land.
How much the oceans will rise isn't clear. The polar ice sheets (the miles-thick shields of ice mantling the continent of Antarctica and the island of Greenland) are difficult places to study because they are remote and harsh.
Scientific discussion has long centered on the need to balance two opposing effects of climate warming: Warmer air and ocean water will cause more melt, adding to sea level, but warmer air will also increase snowfall, reducing sea level.
In the past few years, for the first time, we have good direct measurements of changes in the ice sheet surfaces. As expected, they show some thickening of the interiors of both Greenland and Antarctica due to increased snowfall. But thinning of the ice sheet margins is much stronger, driven by increased melt rates and increased ice flow.
In balance, the ice sheets are losing mass and causing sea level to rise. Most disturbing, the rate of thinning in parts of coastal Greenland and Antarctica considerably exceeds our best-guess predictions.
On top of this, rising sea level from thermal expansion of ocean waters is nearly inevitable. The world's coastlines appear headed for some very bad times.
When we look at the complex environmental systems of our planet, from climate to the polar ice sheets, there will always be lingering uncertainties, and some surprises probably await us. But in the thorough and convincing rebuttal of the last contrary arguments, we have just witnessed a historically important validation of the scientific evidence for human causation of climate warming. And Katrina and the waning polar ice caps remind us how important this is.
It is time for remaining skeptics to look at the tear-streaked faces of refugees from New Orleans, as well as the startling map of ice shrinkage around the North Pole, and begin to plan for the future.
Kurt M. Cuffey is a professor of geography at UC Berkeley. Contact us at [email protected].
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2005/10/09/ING5FF2U031.DTL&hw=cuffey&sn=001&sc=1000
