In his brilliant, sprawling, fifty-two page 2016 paper Dr. James Hansen told the story of what happened at the end of the last interglacial period about 120,000 years ago when a surge of melting of Antarctica’s glaciers caused a sudden climate shift. Paradoxically, the weather around Antarctica got very cold while the bases of Antarctica’s glaciers were melted from below by warm intermediate ocean waters hundreds of feet below the sea surface. The layer of cold fresh glacial melt water insulated the southern ocean around Antarctica slowing heat loss to space in the long southern winter. Thus, rapid melting of Antarctic glaciers was coupled with the rapid build up of heat in the global oceans. The key to understanding this problem is knowing that the enormous mass of Antarctica’s glaciers has pushed the edge of the continent down hundreds of feet (or meters) below sea level allowing warm intermediate ocean waters direct contact with the base of glaciers during periods of rapid global warming.
Hansen used satellite measurements to determine that the earth is taking in substantially more heat than it is radiating back to space in his landmark 2011 paper. Greenhouse gases, in particular CO2, carbon-dioxide and CH4, methane, are trapping increasing amounts heat at the top of the lower atmosphere. The large majority of the heat Hansen calls “the earth’s energy imbalance” is going into warming the oceans. Only one twenty-fifth of the added heat is going into warming the land.
The oceans have an extraordinary heat capacity. Therefore, although there is a rapid climate response to events such as El Niño, the sudden release of heat stored near the surface of the equatorial Pacific ocean, there is also much slower climate response that may take several hundreds of years. Thus the climate warming we have seen to date is like the part of the iceberg that is above water. The warming that is locked in for present greenhouse gas levels is larger than the warming we have seen to date because cool upper ocean waters have mixed to the surface, slowing the rise in sea surface temperatures and air temperatures.
The researchers found that the low range of temperature increase — between 1 and 3 degrees Celsius — offered by historical observations did not take into account long-term warming patterns. When these patterns are introduced, the researchers found that not only do temperatures fall within the canonical range of 1.5 to 4.5 degrees Celsius but that even higher ranges, perhaps up to 6 degrees, may also be possible.
Surprisingly, however, it isn’t the increasing ocean heat content that has made the largest contribution to the increasing intensity of Atlantic hurricanes. The excellent article in the Guardian by Dr. Michael Mann on how climate change made Hurricane Harvey more destructive did not discuss this important effect. It turns out that the small fraction of heat that has gone into warming the atmosphere is very important to the maximum potential tropical storm intensity.
Warming of the lower tropical atmosphere, caused by increasing amounts of heat trapped by greenhouse gases has increased the flow of air from the weather layer of atmosphere (the troposphere) into the stratified layer above it (the stratosphere). This increased flow of tropical tropospheric air into the stratosphere has had a large impact on the maximum possible power of tropical storms.
When I was a geochemistry graduate student at UCLA, eating lunch at weather map discussions in the meteorology department, the physics of hurricanes was not well understood. In my third year, a very young new professor, who just got his PhD from MIT, showed up at map discussions. The rumor passed around that MIT didn’t want to let him go because he was so brilliant, but they thought he needed some experience at another university. The rumor was true. He was quickly hired back by MIT where he developed the modern theory of the physics of hurricanes and thermodynamic storms. Despite being a rising star in theoretical atmospheric science, he was a weather geek just like the rest of us. Of course, he was more rigorous than we could ever be.
Dr Kerry Emanuel found, other factors held constant, that the maximum possible intensity of a hurricane was a function of the water temperature at the base of the hurricane and the outflow temperature at the top of a hurricane. A large, strong hurricane spins up water from hundreds of feet below the surface so sea warm surface temperatures will not maintain an intense hurricane if the warm layer is shallow, unless the hurricane is moves very quickly over the surface. A stalled tropical storm will generally weaken quickly in response to cooling water temperatures unless the warm water layer is thick.
Surprisingly, however, Dr Emanuel discovered that the increasing hurricane intensity in the Atlantic basin was more affected by the cooling of the tropical transition layer (TTL) of the lower stratosphere than it was by the increasing ocean heat content. Lower stratospheric cooling, not well predicted by major climate models, lowered the outflow temperature at the top of the hurricane, increasing the potential power of an Atlantic tropical storm. Improved measurement and analysis of satellite data has done a better job of separating data from the upper troposphere from the lower stratosphere, resolving climate skeptics’ questions about the apparent slow rise in satellite measurements of upper tropospheric temperatures. The University of Alabama upper tropospheric temperature analyses were contaminated by the cooling lower stratosphere. Moreover, lower stratospheric cooling was larger than predicted by climate models. This cooling has apparently made the largest impact of any factor on the increasing intensity Atlantic hurricanes, according to Dr. Emanuel.
Scientists and engineers will be studying Hurricane Harvey for years. I suspect that Dr Mann’s analysis that emphasized the effects of high oceanic heat content will be validated. Hurricane Harvey never got close to its maximum potential intensity. It’s extremely rapid intensification would have been affected by the lower stratospheric cooling at the top of the storm, but what made Harvey so destructive was its continued rainfall and strength after it stalled over coastal Texas. Harvey stayed so strong and destructive because a ring that broke off the loop current in the Gulf of Mexico had rotated offshore of Texas and northern Mexico. In fact, the heat content of the whole Gulf of Mexico has been much higher than normal this year although sea surface temperatures have been only modestly above normal. They key to the increase in the number of extreme rainfall events like Harvey is the energy imbalance Dr, Hansen has written about. That energy imbalance has led to a large increase in heat stored in the upper ocean while sea surface and air temperatures have increased modestly. Persistent winds that would have brought up cool ocean water in decades past are welling up warm waters stored hundreds of feet deep. Those warm waters are fueling record rainfall events in slow moving storms.
Oceanic Heat August 24, 2005 before Katrina
Oceanic Heat August 24, 2017 before Harvey
Please note that for these AOML images of ocean heat than any area with a light blue to yellow to red to white has a high heat content. The heat content in the western Caribbean has been extraordinarily high this year.