occurring to what is the rate of global warming, and what should be done about it. Hansen has continued the development and diagnostics of climate models. For instance, he has helped in the investigations of the decadal trends in tropopause height, which could be a useful tool for determining the human "fingerprint" on climate. , the current version of the GISS model is Model E. This version has seen improvements in many areas, including upper-level winds, cloud height, and precipitation. This model still has problems with regions of marine stratocumulus clouds. A later paper showed that the model's main problems are having too weak of an ENSO-like variability, and poor sea ice modeling, resulting in too little ice in the Southern Hemisphere and too much in the Northern Hemisphere. Climate forcings, feedbacks, and sensitivity In 2000 Hansen authored a paper called "Global warming in the twenty-first century: an alternative scenario" in which he presented a more optimistic way of dealing with global warming, focusing on non-CO2 gases and black carbon in the short run, giving more time to make reductions in fossil fuel emissions. He notes that the net warming observed to date is roughly as big as that expected from non-CO2 gases only. This is because CO2 warming is offset by climate-cooling aerosols emitted with fossil fuel burning and because at that time non-CO2 gases, taken together, were responsible for roughly 50% of the anthropogenic greenhouse gas warming. In a 2007 paper, Hansen discussed the potential danger of "fast-feedback" effects causing ice sheet disintegration, based on paleoclimate data. George Monbiot reports "The IPCC predicts that sea levels could rise by as much as this century. Hansen's paper argues that the slow melting of ice sheets the panel expects doesn't fit the data. The geological record suggests that ice at the poles does not melt in a gradual and linear fashion, but flips suddenly from one state to another. When temperatures increased to 2–3°C (3.6–5.4°F) above today's level 3.5 million years ago, sea levels rose not by 59 centimeters but by . The ice responded immediately to changes in temperature." Hansen stressed the uncertainties around these predictions. "It is difficult to predict time of collapse in such a nonlinear problem … An ice sheet response time of centuries seems probable, and we cannot rule out large changes on decadal time-scales once wide-scale surface melt is underway." He concludes that "present