Riders on the storms

By Bryan Walker 06/03/2011

I listened with interest to Kevin Trenberth on the latest Climate Show describing how the increased water vapour in the atmosphere resulting from human-caused global warming is leading to greater extremes in weather events. It sent me back to take another look at the section in James Hansen’s book Storms of My Grandchildren where he explains the greatly increased strength of storms we can expect as the century unfolds, unless we leave most fossil carbon in the ground. I reviewed the book a while back on Hot Topic and thought it worth outlining more closely here, as an extension of my review, Hansen’s argument in the ten pages where he specifically addresses the storms of which the book’s title speaks.

As ice sheet disintegration begins in earnest he writes of a chaotic transition period in which our grandchildren will live the rest of their lives. Ice sheet disintegration in Earth’s past needed millennia, but human forcing is so much more powerful than natural forcings that ice sheets will respond much more rapidly.  Currently most of the recent energy imbalance due to increased greenhouse gases in the atmosphere is warming the ocean and only a small fraction is being used to melt ice. This division of excess energy will shift more to ice melt as the ice sheets are softened up and begin to discharge ice to the ocean more rapidly.

Increased ice discharge in both West Antarctica and Greenland will cool the neighbouring ocean. Although Greenland is not as vulnerable to rapid collapse as West Antarctica is it can lose mass fast enough to influence North Atlantic Ocean surface temperature. This freshwater melt will also decrease the salinity and hence the density of the water, making it less able to sink to the ocean bottom as it currently does, where it feeds the ocean ’conveyor’ circulation and allows warmer water to move north to replace it. If that deepwater formation slows down the regional North Atlantic cooling from ice melt will be enhanced.

Meanwhile in low latitudes the atmosphere and the ocean surface will be getting warmer and warmer as the century proceeds. This will exacerbate trends already apparent such as mountain glacier melting, expansion of dry sub-tropical regions, more intense forest fires and competition for diminishing freshwater supplies. There will be increased desiccation but in other times and places heavier rain and increased floods.

The greatest impact warming will have on storms is through the increase in water vapour it causes. Atmospheric water vapour increases rapidly with only a small temperature rise. Latent heat is the energy acquired by water vapour when it evaporates. When it condenses that latent energy is released as heat that is potentially available to fuel a storm. The storm types driven by latent heat include thunderstorms, tornadoes, and tropical storms such as hurricanes and typhoons. The greater availability of this heat will mean that the strength of the strongest storms will increase as global warming increases. There will also be an expansion of the regions liable to severe storms.

This is just the beginning. There are three ratcheting effects in waiting. One is the development of more powerful and destructive mid-latitude or frontal cyclones. They depend on the temperature difference between the cold and warm air masses as well as on the amount of moisture in the atmosphere behind the warm front. The melting ice sheets will exacerbate this once they begin to disintegrate rapidly enough to keep regional ocean surface temperature from rising as fast as continental temperatures and temperatures at lower latitudes. Increased moisture content in lower- and mid-latitude warm air co-existing with ice-cooled polar air masses will increase the intensity of frontal cyclones.

The consequences of even a metre of sea level rise combined with increased storm strength are horrendous to contemplate.

The second ratcheting is far greater. It occurs when the ice sheets’ rapid disintegration causes a sea level rise measured in metres. Eventually ice sheets begin to disintegrate at rates of several metres of sea level rise per century. We could soon create conditions that guarantee this happening, but it is likely to be several decades before a rapid sea level rise begins. Hansen notes that we have been surprised by how fast some other climate changes have occurred, but for the moment offers his best estimate of when large sea level change will begin as during the lifetime of his grandchildren. The consequences of even a metre of sea level rise combined with increased storm strength are horrendous to contemplate. He offers a few examples of what it will mean for vulnerable places in various parts of the world.

The third ratcheting effect would be the melting of methane hydrates. Of greatest concern are those in sediments on the ocean floor, because of their great volume. Hansen relates the chance of their melting to possible ocean circulation changes because of the freshwater additions to both the North Atlantic and Antarctic Oceans through ice sheet disintegration. Global ocean circulation reorganised during the Paleocene-Eocene thermal maximum about 54 million years ago when a sudden large global warming occurred and deep water formation took place in the Pacific rather than the North Atlantic, flooding the ocean floor with warmer Pacific Ocean water. If that happens again, melting methane hydrates, there will be no plausible way for humans to reverse the change of ocean circulation. The released methane added to the high levels of carbon dioxide will result in a huge planetary energy imbalance and the remaining ice on the planet will disappear. That means a sea level rise of 75 metres.

Hansen, as usual, communicates the science to his readership with clarity and fully appropriate urgency.  These are not remote consequences he is exploring, but at least strong possibilities and in some cases inescapable certainties. No lay reader prepared to take a little time to come to terms with the concepts can fail to understand the serious risks we run if we carry on exploiting fossil fuels. No policy maker can claim not to have been made aware of the danger of continuing on our present course.

Hansen is not off on some flight of fancy of his own. He is interpreting solid mainstream science. It’s from that base that he says towards the end of the book that our planet is in imminent danger of crashing and that the fight for effective policies to prevent that is the most urgent fight of our lives.

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