«DIRECT TESTIMONY OF JAMES E. HANSEN Q. Please state your name and business address. A. My name is James E. Hansen. My business address is 2880 ...»
A. We can project future global warming with reasonable confidence, for different assumed scenarios of greenhouse gases, by extending the climate model simulations that matched well the observed global temperature change in the past century. Figure 16 shows such a projection based on the GISS global climate model, which has climate sensitivity close to 3°C for doubled CO2. The model excludes slow climate feedbacks such as changes of ice sheet area and global vegetation distributions, but the effects of those slow feedbacks on global mean temperature should be small during the next several decades.
‘Business-as-Usual’ climate scenarios, such as IPCC scenarios A1B and A2, yield additional global warming of at least 2°C in the 21st century. Actual warming for ‘business-asusual’ climate forcing could be larger because: (1) slow climate feedbacks such as ice sheet disintegration, vegetation migration, and methane release from melting permafrost are not included, (2) atmospheric aerosols (small particles, especially sulfates) that have a cooling effect are kept fixed, but it is expected that they could decrease this century, (3) CO2 emissions as high 20 as in business-as-usual scenarios may have climate effects large enough to alter the ability of the biosphere to take up the assumed proportion of CO2 emissions.
The ‘alternative scenario’ is defined with the aim of keeping additional global warming, beyond that of 2000, less than 1°C. This requires that additional climate forcing be kept less than about 1.5 W/m2, assuming a climate sensitivity of about 3°C for doubled CO2, and in turn this requires that CO2 be kept from exceeding about 450 ppm, with the exact limit depending upon how well other climate forcings are constrained, especially methane (Hansen et al. 2000).
Figure 16 shows that additional global warming in the alternative scenario is about 0.8°C by 2100, and it remains less than 1°C under the assumption that a slow decrease in greenhouse gas forcing occurs after 2100.
Q. How do these levels of global warming relate to dangerous climate change?
A. That is the fundamental issue, because practically all nations, including the United States, have signed the Framework Convention on Climate Change, agreeing to stabilize greenhouse gas emissions at a level that prevents “dangerous” anthropogenic interference with the climate system (Figure 17). In just the past few years it has become clear that atmospheric composition is already close to, if not slightly beyond, the dangerous level of greenhouse gases. In order to understand this situation, it is necessary to define key metrics for what constitutes “danger”, to examine the Earth’s history for levels of climate forcing associated with these metrics, and to recognize changes that are already beginning to appear in the physics of the climate system.
Principal metrics defining dangerous include: (1) ice sheet disintegration and sea level rise, (2) extermination of species, and (3) regional climate disruptions (Figure 18). Ice sheet disintegration and species extinction proceed slowly at first but have the potential for disastrous non-linear collapse later in the century. The consequences of ice sheet disintegration and species extinction could not be reversed on any time scale of interest to humanity. If humans cause multi-meter sea level rise and exterminate a large fraction of species on Earth, they will, in effect, have destroyed creation, the planet on which civilization developed over the past several thousand years.
Regional climate disruptions also deserve attention. Global warming intensifies the extremes of the hydrologic cycle. On the one hand, it increases the intensity of heavy rain and floods, as well as the maximum intensity of storms driven by latent heat, including thunderstorms, tornados and tropical storms. At the other extreme, at times and places where it is dry, global warming will lead to increased drought intensity, higher temperatures, and more and stronger forest fires. Subtropical regions such as the American West, the Mediterranean region, Australia and parts of Africa are expected to be particularly hard hit by global warming.
Because of earlier spring snowmelt and retreat of glaciers, fresh water supplies will fail in many locations, as summers will be longer and hotter.
Q. Is it possible to say how close we are to deleterious climate impacts?
A. Yes. I will argue that we are near the dangerous levels for all three of these metrics.
In the case of sea level, this conclusion is based on both observations of what is happening on the ice sheets today and the history of the Earth, which shows how fast ice sheets can disintegrate and the level of warming that is needed to spark large change.
Figure 19 shows that the area on the Greenland ice sheet with summer melt has been increasing over the period of satellite observations, the satellite view being essential to map this region. The area with summer melt is also increasing on West Antarctica.
Q. Is it not true that global warming also increases the snowfall rate, thus causing ice sheets to grow faster?
A. The first half of that assertion is correct. The inference drawn by ‘contrarians’, that global warming will cause ice sheets to become bigger, defies common sense as well as abundant paleoclimate evidence. The Earth’s history shows that when the planet gets warmer, ice sheets melt and sea level increases. Ice sheet size would not necessarily need to decrease on short time scales in response to human-made perturbations. However, we now have spectacular data from a gravity satellite mission that allows us to evaluate ice sheet response to global warming.
The gravity satellite measures the Earth’s gravitational field with sufficient precision to detect changes in the mass of the Greenland and Antarctic ice sheets. As shown by Figure 22, the mass of the ice sheet increases during the winter and decreases during the melting season.
However, the net effect is a downward trend of the ice sheet mass. In the past few years Greenland and West Antarctica have each lost mass at a rate of the order of 150 cubic kilometers per year.
Q. Is sea level increasing at a significant rate?
A. Sea level is now increasing at a rate of about 3.5 cm per decade or 35 cm per century, with thermal expansion of the ocean, melting of alpine glaciers, and the Greenland and West Antarctic ice sheets all contributing to this sea level rise. That is double the rate of 20 years ago, and that in turn was faster than the rate a century earlier. Previously sea level had been quite stable for the past several millennia.
Q. Is the current level of sea level rise dangerous?
A. This rate of sea level rise is more than a nuisance, as it increases beach erosion, salt water intrusion into water supplies, and damage from storm surges. However, the real danger is the possibility that the rate of sea level rise will continue to accelerate. Indeed, it surely will accelerate, if we follow business-as-usual growth of greenhouse gas emissions.
Q. How fast can sea level rise and when would rapid changes be expected?
A. Those questions are inherently difficult to answer for a non-linear process such as ice sheet disintegration. Unlike ice sheet growth, which is a dry process limited by the rate of snowfall, ice sheet disintegration is a wet process that can proceed rapidly and catastrophically once it gets well underway.
Some guidance is provided by the Earth’s history. When the Laurentide ice sheet, which covered Canada and reached into the northern edges of the United States, disintegrated following the last ice age, there were times when sea level rose several meters per century. The Greenland and West Antarctic ice sheets are at somewhat higher latitudes than the Laurentide ice sheet, but West Antarctica seems at least as vulnerable to rapid disintegration because it rests on bedrock below sea level. Thus the West Antarctic ice sheet is vulnerable to melting by warming ocean water at its edge as well as surface melt. In addition, if we follow business-as-usual, the humanmade climate forcing will be far larger and more rapid than the climate forcings that drove earlier deglaciations.
I have argued (Hansen 2005, 2007a) that business-as-usual greenhouse gas growth almost surely will cause multi-meter sea level rise within a century. High latitude amplification of global warming would result in practically the entire West Antarctic and Greenland ice sheets being bathed in meltwater for a lengthened melt season. A warmer ocean and summer rainfall could speed flushing of the ice sheets. If we wait until rapid disintegration begins, it will be impossible to stop.
Q. What consequences would be expected with multi-meter sea level rise?
A. Most of the world’s large cities are on coast lines (Figure 23). The last time that global mean temperature was 2-3°C warmer than now was in the Pliocene, when sea level was about 25 meters higher than today. About one billion people live within 25-meter elevation of sea level.
As shown by Figure 24, most East Coast cities in the United States would be under water with a sea level rise that large, almost the entire nation of Bangladesh, the State of Florida, and an area in China that presently contains about 300 million people. There are historical coastal cities in most countries. A sea level rise of 5-7 meters, which could be provided by West Antarctica alone, is enough to displace a few hundred million people.
Q. Does sea level provide a precise specification of ‘dangerous’ warming?
A. I suggest that it is useful to look at prior interglacial periods, some of which were warmer than our current interglacial period. In some of these periods, e.g., the interglacials ~125 and ~425 thousand years ago, sea level was higher than today by as much as a few meters, but sea level did not approach the level in the Pliocene. Although we do not have accurate measurements of global mean temperature for the earlier interglacial periods, we do have local measurements at places of special relevance.
Figure 25a is the temperature in the Western Pacific Warm Pool, the warmest ocean region on the planet, a region of special importance because it strongly affects transport of heat to higher latitudes via both the atmosphere and ocean. Figure 26b is the temperature in the Indian Ocean, the place that has the highest correlation with global mean temperature during the period of instrumental data, the period when an accurate global mean temperature can be calculated (Hansen et al. 2006). Figure 25 concatenates modern instrumental temperatures with proxy paleo measures. In both of these regions it appears that the warming of recent decades has brought recent temperatures to within about 1°C or less of the warmest interglacial periods.
Tropical ocean temperature change is only moderately smaller than global mean temperature change in both recent times and glacial-interglacial climate change. For this reason, I assert that it would be foolhardy for humanity to allow additional global warming to exceed about 1°C.
Q. But if additional global warming is kept less than 1°C that does not seem to guarantee that sea level rise of a few meters would not occur, given the changes that occurred in the previous interglacial periods, does it?
A. You are right, and I am not recommending that the world should aim for additional global warming of 1°C. Indeed, because of potential sea level rise, as well as the other critical metrics that I will discuss, I infer that it is desirable to avoid any further global warming.
23 However, I also note that there is an enormous difference between global warming less than 1°C and global warming of 2-3C. The latter warming would have the global climate system pointed toward an eventual sea level rise measured in the tens of meters. In that case we should expect multi-meter sea level rise this century and initiation of ice sheet disintegration out of our control with a continually rising sea level and repeated coastal disasters unfolding for centuries.
Economic and social consequences are difficult to fathom.
With global warming less than 1°C it is possible that sea level rise this century would be less than 1 meter. Ice sheet changes would likely unfold much more slowly than with 2-3°C global warming. If the maximum global warming is kept less than 1°C, it may be practical to achieve moderate adjustments of global climate forcings that would avert the occurrence of large sea level change. Human-made gases in the air will decrease when sources are reduced sufficiently, so as events unfold and understanding improves, it may prove necessary to set goals that yield a declining global temperature beyond the human-induced maximum temperature.
However, considering the 1000-year lifetime of much of the CO2, if the additional warming is 2°C, it will be impractical to avoid disastrous consequences.
Q. What other ghosts of climate future can be seen?
A. Another potential consequence that would be irreversible is extermination of species. Animal and plant species can survive only within certain climatic zones. As climate changes, animals and plants can migrate, and in general they deal successfully with fluctuating climate. However, large climate changes have caused mass extinctions in the past. Several times in the Earth’s history global warming of five degrees Celsius or more led to extinction of a majority of species on the planet. Of course other species came into being over many thousands of years. But mass extinctions now would leave a far more desolate planet for as long as we can imagine.
Global warming of 0.6°C in the past three decades has initiated a systematic movement of climatic zones, with isotherms moving poleward at a rate of typically 50-60 km per decade (Hansen et al. 2006). As this movement continues, and as it would accelerate with business-asusual increases of fossil fuel use, it will add a strong climatic stress to the other stresses that humans have placed on many species. Species at high latitudes (Figure 26) and high altitudes (Figure 27) are in danger of, in effect, being pushed off the planet by global warming. Many other species will be threatened as the total movement of climatic zones increases, because some species are less mobile than others. Interdependencies of species leave entire ecosystems vulnerable to collapse.