The following is a response to Tuesday’s column entitled “Combating the spread of green hysteria” by Jordan Harp.

If the Earth’s atmosphere did not contain water vapor, carbon dioxide (CO2) and ozone, which make up less than 0.25 percent of its composition, the average surface temperature would be zero degrees – so cold there would be no liquid water and, thus, no life. The fact that the temperature is a life-supporting 60 degrees is due to these seemingly minor greenhouse gases (GHG). On Venus the atmosphere is all CO2, the surface pressure is 90 times that on Earth, and the greenhouse warming is 900 degrees, about twice as hot as your home oven can get. The natural greenhouse effect is indisputable.

Humanity has added CO2 to the atmosphere by burning fossil fuels (coal, oil and natural gas) and by deforestation since the beginning of the industrial revolution. This added CO2 decreased the radiation emitted out to space by the Earth such that it was less than the radiation absorbed by the Earth from the sun. This caused the Earth’s surface and lower atmosphere to warm towards restoring the balance. This is the human-caused greenhouse effect.

Starting with the industrial revolution until now the United States and Europe added most of the CO2 to the atmosphere. Soon this role will pass to the developing countries, especially China and India.

There are seven times as many people in China as in the United States, each of whom, on average, uses one-sixth as much energy as a person in the U.S. If these countries develop as did the U.S., then they alone will emit 42 times as much CO2 as the U.S., which will be 10 times the emission by the entire world. Clearly, reducing the emission of GHG is a geopolitical problem of unprecedented scope.

I have analyzed the record of average surface-air temperature from the mid-19th century to the end of the 20th century, using a simple mathematical climate model to simulate this quantity for many different possible causes. This analysis showed that the human-caused greenhouse effect was the predominant cause of the observed warming, with another contribution by the natural oscillation of the Atlantic thermohaline circulation.

Today there are many worrisome signs in greenhouse Earth, especially the loss of ice in the Greenland ice sheet, and the complete loss of the Larsen B ice shelf in West Antarctica. Clearly the climate is changing significantly because of the human-caused greenhouse effect.

I have also found that for a moderate warming by the end of this century, the countries that emit CO2, located in temperate and high latitudes, slightly benefit in terms of market impacts, especially agriculture. But countries that emit little or no CO2, located in tropical latitudes, particularly in Africa, are harmed by a reduction in their gross domestic product. Clearly this is not equitable and reverses the “Polluter Pays Principle” of international environmental law. Furthermore, for end-of-century warming that is not moderate, all countries are harmed.

I have calculated the expected values of the changes in average surface-air temperature and sea level this century for four possible future emission scenarios absent climate policy, three possible climate sensitivities , and two possible melt rates for the Greenland ice sheet, each weighted by a subjective probability.

The results in 2100 are 4 degrees and 1.6 feet, respectively, both relative to year 2000. Even more disquieting, both temperature and sea level continue to increase into the 22nd century.

I also calculated the effect of four “1 percent solutions” wherein either the global or regional-only emissions of greenhouse gases are reduced linearly to zero by 1 percent per year beginning in 2010. Only for the global “1 percent solution” does the temperature increase level off by 2100, albeit the sea-level rise does not. This shows the importance of having the entire world reduce its emission of GHG.

I have examined the effect of hedging against our uncertain climate future by imposing a tax on carbon now compared to 30 years from now, in both cases the tax increasing annually at the then rate of interest. Results were obtained over two uncertain quantities, the climate sensitivity and the maximum allowed global warming. Imposing the tax now not only holds open options that are foreclosed by delaying the tax by 30 years, the adjustment cost is less for the current tax than for the tax delayed by 30 years. Thus uncertainty is not a justification for doing nothing now, rather it is the reason for acting now.

Another uncertainty of our future climate is whether or not there are tipping points, such as a shutdown of the Atlantic thermohaline circulation (7) and the loss of the Greenland ice sheet, and, if they exist, how close we are to them. The only way to know for certain is after the fact of crossing one. But clearly by then it will be too late to mitigate that crossing, and instead we will have to suffer and adapt to its climatic consequences.

Thus, the most important climate factor to hedge against is the crossing of such uncertain tipping points. To do this we must make the transition this century, as quickly as we can, from the Greenhouse-Gas-Emission Age to the Post-Greenhouse-Gas-Emission Age. To not do so would be to play Russian roulette with the Earth’s climate.

What can you do to reduce your “carbon footprint”? You can go to the Web site of the Oregon State University Campus Carbon Challenge (http://oregonstate.edu/~johnsonc/50%20Things.html).

There you will find “50 ways to reduce your carbon habit” grouped into eight categories: On the Go, At Home, In the Kitchen, In the Bathroom, Don’t Trash It, While Shopping, At the Office and Spread the Word. It’s up to each of us to do our part to “Stop Global Warming Now.”

Michael Schlesinger is a professor in the Department of Atmospheric Sciences and Head of the Climate Research Group. He is a recipient of the 2007 Nobel Peace Prize.