Global warming IS a time bomb.
There may still be time to defuse it, but that requires policy-makers to take the actions that are needed, not the ineffectual actions they are discussing.
Despite the publicity that global warming has received, there is a large gap between what is understood by the relevant scientific community, and what is known by the people who need to know, the public and policymakers. Global warming is small compared to day-to-day weather fluctuations, so it is hard for people to recognize that we have a crisis – but we do.
The climate system has great inertia, caused, e.g., by the 4-kilometer-deep ocean and the thick ice sheets on Antarctica and Greenland, which have only partly responded to the human-made changes of atmospheric composition. That inertia is not our friend. It is a Trojan horse. By the time the public notices that change is underway the momentum of the climate system may be sufficient to guarantee much larger changes. The climate system can pass tipping points, such that large change continues out of our control.
The bad news is that we have already passed into a dangerous range of atmospheric carbon dioxide.
The good news is that if we act smart and promptly it is still feasible to achieve a safe level of atmospheric gases, and the actions needed to achieve that would have multiple benefits in addition to climate stability.
There are several climate tipping points of special concern.
Tipping points are "non-linear" phenomena, which means that they can reach a point at which rapid catastrophic change occurs. It is inherently difficult to determine the time at which non-linear collapse will occur, even in cases where such rapid change is certain.
The mechanism that seems to be most important for disintegration of the great ice sheets that cover Antarctica and Greenland begins with ocean warming. Ocean warming leads to melting of ice shelves, which are tongues of ice that stretch out into the ocean. The ice shelves buttress the ice sheets, so when ice shelves disappear, the more mobile parts of the ice sheet, the ice streams, can surge into the ocean. Thus, removal of the ice shelves is somewhat akin to taking the cork out of a bottle — it allows the material behind to flow rapidly.
We know from Earth's history that once ice sheet disintegration is well underway, sea level can rise by several meters per century.
Let's look more at processes contributing to ice sheet disintegration.
We have accurate satellite measurements of the area on Greenland with summer melting on the snow and ice surface. The area with melting fluctuates from year to year, depending on year-to-year weather fluctuations, but there is a long-term increase of the melt area.
The area with summer melting is shown by the red area, with maps for 1992 and 2007, the years with the least and most melt area. The trend has an increase of 50 percent in melt area during the past three decades.
The meltwater runs to a low spot on the ice surface, where it burrows a hole in the ice sheet that carries the water all the way to base of the ice sheet.
There, the meltwater lubricates the base of the ice sheet, accelerating the discharge of giant icebergs to the ocean.
This is one of the processes causing ice loss from Greenland and Antarctica to increase.
Another process, probably even more effective than surface melt, is melting of ice shelves by warmer ocean water.
Ice shelves are tongues of ice extending from the large ice sheets into the ocean. They buttress the large ice sheet, helping to keep it in place. As a warming ocean melts the ice shelves, icebergs begin to stream more rapidly into the ocean. Until recently, some scientists argued that global warming may cause ice sheets to grow, because a warmer atmosphere holds more water vapor and thus produces greater winter snowfall. Of course, common sense suggests that the ice sheets will become smaller as the planet becomes warmer, but accurate measurements are needed to prove what is happening.
Beginning in 2002, precise measurements of Earth's gravitational field were made by the NASA GRACE satellite. The gravity data yield accurate measurements of changes of the mass of the ice sheets. Overall, Greenland is losing mass at a substantial rate. Earlier this decade, it was losing nearly 200 gigatons per year; in the past few years, that loss has increased to almost 300 gigatons per year. One gigaton is the mass of one cubic kilometer of water.
The status of the Antarctic ice sheet is even more important.
The West Antarctic ice sheet is vulnerable to possible rapid disintegration because much of that ice sheet is grounded on bedrock well below sea level. A warming ocean could potentially dislodge its ice more readily than ice that is sitting on land above sea level. In the past few years the rate of its mass loss has increased, with the recent rate being close to 200 cubic kilometers per year. The West Antarctic ice sheet contains enough ice to raise sea level by about seven meters.
Earth's history reveals numerous cases in which ice melt caused sea level to rise several meters per century. But if business-as-usual greenhouse gas emissions continue, the human-made climate forcing will be much greater than the natural forcings that caused these earlier ice sheet disintegrations.
I find it implausible that the West Antarctic ice sheet could survive this century if business-as-usual emissions continue. Thus, in such an emission scenario, sea level rise of several meters should be expected this century, with still further sea level rise continuing, out of control of humanity.
Arctic sea ice is another potential tipping point of the climate system.
The area of Arctic sea ice at the end of the summer began to be measured accurately from satellites in the late 1970s. The area of sea ice fluctuates from year to year, based on variable weather patterns. However, overall there has been a decline in sea ice area over the past three decades.
In 2007 there was a sharp decline of sea ice area, to an amount just over half of the ice area three decades earlier. Although sea ice recovered slightly in 2008 and even more in 2009, most analyses indicate that all summer sea ice will be lost within the next few decades if business-as-usual greenhouse gas increases continue.
It is difficult to imagine that the Greenland ice sheet could survive if the Arctic sea ice disappears in summer.
Stabilization of Arctic sea ice requires, to first approximation, that Earth's energy balance is restored.
At present, because human-made greenhouse gases have reduced the amount of heat radiation that Earth is emitting to space, our best estimate is that the planet is out of balance by about
one-half watt per square meter, with uncertainty of about one-quarter
watt per square meter.
Other things being equal, the amount of carbon dioxide in the air would need to be reduced from its present 387 ppm (parts per million) to about 350 ppm in order to increase emission of heat radiation to space by 0.5 watts per square meter and restore Earth's energy balance.
Conceivably it will be necessary to return carbon dioxide even closer to its pre-industrial value of 280 ppm, especially for matters such as ice sheet stability and sea level. However, it is probably undesirable to return carbon dioxide fully to its pre-industrial level, as humans have also introduced negative (cooling) climate forcings, via atmospheric aerosols and changes of surface reflectivity.
There is no practical need to define the carbon dioxide target more precisely now. The goal "less than 350 ppm" already tells us all that is needed for policy purposes for the foreseeable future.
We must take actions to return atmospheric carbon dioxide from its present level of 387 ppm (annual global mean for 2009) to an amount no greater than 350 ppm if we wish to preserve a planet similar to the one we inherited from our elders, the planet on which civilization developed, a planet with stable shorelines and climate patterns like those for which our infrastructure has been developed.
Adapted from "Global Warming Time Bomb: Actions Needed to Avert Disaster," presented in Amsterdam on Oct. 26, 2009.