Of the many points to be made, here are a few Koonin’s analysis totally fails to consider the significant risk of very serious impacts on marine life of ocean acidification from the rising  concentration. Impacts are already affecting those growing oysters and other shelled organisms in the Pacific Northwest, and coral atolls around the world are at risk over coming decades—and that is pure chemistry totally independent of climate models Koonin’s point that the climate has changed so much in the past is actually one of the key reasons to be worried about human-induced climate change. Were the past climate stable even as the various natural forcings were changing, then there would be less reason for concern that human-induced forcings could change the climate. But reality is that past natural forcings caused significant changes in the climate—and now human activities are leading to forcings that are comparable to or even larger than natural ones in the past. In addition, the forcings being created will change climate more rapidly than have natural factors, making this unprecedented except for the catastrophic changes that have followed the impacts of large asteroids. Thus, contrary to Koonin’s assertion that past climate change suggests a policy of caution, a more appropriate conclusion would be that insights gained from past climate change should be leading to much more aggressive policy action than is now underway Yes, there is lots more to be learned, but the basic physics of the climate change issue have been clear since the 1960s when the President’s Science Advisory Council sent their report to President Johnson and Congress in 1965. Except for refrigerants of various types, human activities are adding increasing amounts of carbon dioxide (and other substances) to the atmosphere that are amplifying the natural warming effect of these substances in the atmosphere—it is not that we are not familiar with the substances we are adding to the atmosphere and have dealt with for decades. That these gases will cause warming has been recognized since the mid-19th century and adding more will surely cause more warming. Certainly, there are more questions to be investigated and resolved, and they do affect how best to adapt and other policies—but they do not alter at all the fundamental reality that human activities have become the primary driver of changes in climate, overwhelming the cycling changes in solar radiation and being much longer lasting than the occasional volcanic eruptions On sea level rise, Koonin’s comments are again mistaken and misleading. The present rate of sea level rise is well above the rate for the first half of the 20th century. The relatively stable climate that has allowed civilization to expand over the last several thousand years has kept sea level quite constant—you can still visit the coast of Sicily and find the remains of salt flats constructed in ancient times. That climate change can cause sea level to change is a key lesson from past changes in the climate that Koonin fails to mention. Since the peak of the last glacial cycle, sea level rose about 20 meters for each one degree Celsius increase in global average temperature (equivalent to a sea level rise of about 30 feet per degree Fahrenheit!!) While it took centuries or more for the full effect to be felt in the past when natural forcings were changing slowly, the adjustment will likely be much more rapid with the faster increase in forcing due to human activities. While the rate of rise will likely decrease slowly with warming because there is less ice on land to be melted, there are still about 75 meters (near 250 feet) of potential sea level rise in the ice tied up in the Greenland and Antarctic ice sheets, which have been experiencing accelerated loss of mass.