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«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 ...»

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A. As the reality of climate change becomes more apparent, as the long-term consequences of further climate change are realized, and as the central role of coal in determining future atmospheric CO2 is understood, the pressures to use coal only at power plants where the CO2 is captured and sequestered will increase. If the public begins to stand up in a few places and successfully opposes the construction of power plants that burn coal without capturing the CO2, this may begin to have a snowball effect, helping utilities and politicians to realize that the public prefers a different path, one that respects all life on the planet.

The changes in behavior will need to run much broader and deeper than simply blocking new dirty coal plants. Energy is essential to our way of life. We will have to find ways to use energy more efficiently and develop renewable and other forms of energy that produce little if any greenhouse gases. The reward structure for utilities needs to be changed such that their profits increase not in proportion to the amount of energy sold, but rather as they help us achieve greater energy and carbon efficiency. As people begin to realize that life beyond the fossil fuel era promises to be very attractive, with a clean atmosphere and water, and as we encourage the development of the technologies needed to get us there, we should be able to move rapidly toward that goal. But we need tipping points to get us rolling in that direction.

Iowa, and this specific case, can be a tipping point, leading to a new direction. A message that ‘old-fashioned’ power plants, i.e., those without carbon capture and sequestration, are no longer acceptable, would be a message of leadership, one that would be heard across Iowa and beyond the state’s borders.

Q. Alleged implications of continued coal burning without carbon capture are profound and thus require proof of a causal relationship between climate change and CO2 emissions.

What is the nature of recent global temperature change?

A. Figure 1(a) shows global mean surface temperature change over the period during which instrumental measurements are available for most regions of the globe. The warming since the beginning of the 20th century has been about 0.8°C (1.4°F), with three-quarters of that warming occurring in the past 30 years.

Q. Warming of 0.8°C (1.4°F) does not seem very large. It is much smaller than day to day weather fluctuations. Is such a small warming significant?

A. Yes, and it is important. Chaotic weather fluctuations make it difficult for people to notice changes of underlying climate (the average weather, including statistics of extreme fluctuations), but it does not diminish the impact of long-term climate change.

First, we must recognize that global mean temperature changes of even a few degrees or less can cause large climate impacts. Some of these impacts are associated with climate tipping points, in which large regional climate response happens rapidly as warming reaches critical levels. Already today’s global temperature is near the level that will cause loss of all Arctic sea ice. Evidence suggests that we are also nearing the global temperature level that will cause the West Antarctic ice sheet and portions of the Greenland ice sheet to become unstable, with potential for very large sea level rise.

Second, we must recognize that there is more global warming “in the pipeline” due to gases humans have already added to the air. The climate system has large thermal inertia, 5 mainly due to the ocean, which averages 4 km (about 2.5 miles) in depth. Because of the ocean’s inertia, the planet warms up slowly in response to gases that humans are adding to the atmosphere. If atmospheric CO2 and other gases stabilized at present amounts, the planet would still warm about 0.5°C (about 1°F) over the next century or two. In addition, there are more gases “in the pipeline” due to existing infrastructure such as power plants and vehicles on the road. Even as the world begins to address global warming with improved technologies, the old infrastructure will add more gases, with still further warming on the order of another 1°F.

Third, eventual temperature increases will be much larger in critical high latitude regions than they are on average for the planet. High latitudes take longer to reach their equilibrium (long-term) response because the ocean mixes more deeply at high latitudes and because positive feedbacks increase the response time there (Hansen et al., 1984). Amplification of high latitude warming is already beginning to show up in the Northern Hemisphere. Figure 1(b) is the geographical pattern of mean temperature anomalies for the first six years of the 21st century, relative to the 1951-1980 base period. Note that warming over land areas is larger than global mean warming, an expected consequence of the large ocean thermal inertia. Warming is larger at high latitudes than low latitudes, primarily because of the ice/snow albedo feedback.

Warming is larger in the Northern Hemisphere than in the Southern Hemisphere, primarily because of greater ocean area in the Southern Hemisphere, and the fact that the entire Southern Ocean surface around Antarctica is cooled by deep mixing. Also human-caused depletion of stratospheric ozone, a greenhouse gas, has reduced warming over most of Antarctica. This ozone depletion and CO2 increase have cooled the stratosphere, increased zonal winds around Antarctica, and thus warmed the Antarctic Peninsula while limiting warming of most of the Antarctic continent (Thompson and Solomon, 2002; Shindell and Schmidt, 2004).

Until the past several years, warming has also been limited in Southern Greenland and the North Atlantic Ocean just southeast of Greenland, an expected effect of deep ocean mixing in that vicinity. However, recent warming on Greenland is approaching that of other landmasses at similar latitudes in the Northern Hemisphere. On the long run, warming on the ice sheets is expected to be at least twice as large as global warming. Amplification of warming at high latitudes has practical consequences for the entire globe, especially via effects on ice sheets and sea level. High latitude amplification of warming is expected on theoretical grounds, it is found in climate models, and it is confirmed in paleoclimate (ancient climate) records.

Q. But those paleoclimate records show that the Earth’s climate has changed by very large amounts many times in the past. For that reason, the NASA Administrator has suggested that we may not need to “wrestle” with human-made climate change. How do you reach a contrary conclusion?

Paleoclimate data, indeed, reveal large climate changes. But that history of ancient climate A.

changes shows that modest forcing factors can produce large climate change. In fact, paleoclimate data provide our most accurate and certain measure of how sensitive global climate is to climate forcings, including human-made climate forcings.

Q. What is a climate forcing?

A climate forcing is an imposed perturbation to the Earth’s energy balance, which would tend to A.

alter the planet’s temperature. For example, if the sun were to become 1% brighter, that would be a forcing somewhat more than +2 W/m2, because the Earth absorbs about 238 W/m2 of energy from the sun. An increase of greenhouse gases, which absorb terrestrial heat radiation and thus 6 warm the Earth’s surface, is also a positive forcing. Doubling the amount of atmospheric CO2 is a forcing of about +4 W/m2.

Q. How large are natural climate variations?

A. That depends on the time scale. A useful time scale to examine is the past several hundred thousand years. There is good data for the temperature, changes of atmospheric composition, and the most important changes on the Earth’s surface. Specifically, we know the amount of long-lived greenhouse gases, CO2, CH4 and N2O, as a function of time from air bubbles in the ice sheets. Ice sheets are formed by snowfall that piles up year by year and compresses into ice as the weight of snow above increases. The date when the snow fell is known accurately for about the past 15,000 years from counting annual layers marked by summer crusting. Annual layers can be clearly distinguished in the upper part of the ice sheet. Less precise ways of dating ice layers are available for the entire depth of the ice sheets. The temperature when the snow flakes fell is inferred from the isotopic composition of the ice.

Figure 2 shows the temperature on the Antarctic ice sheet for the past 425,000 years.

Similar curves are found from Greenland and from alpine ice cores, as well as from ocean sediment cores. Layered ocean sediments contain the shells of microscopic animals that lived in the ocean, the proportion of elements in these microscopic shells providing a measure of the ocean temperature at the time the animals lived. Swings of temperature from warm interglacial periods to ice ages occur worldwide, with the glacial-interglacial temperature range being typically 3-4°C in the tropics, about 10°C at the poles, and about 5°C on global average.

We live today in a warm interglacial period, the Holocene, now almost 12,000 years in duration. The last ice age peaked about 20,000 years ago. Global mean temperature was about 5°C colder than today, with an ice sheet more than a mile thick covering Canada and reaching into the United States, covering the present sites of Seattle, Minneapolis, and New York. So much water was locked in this ice sheet, and other smaller ice sheets, that sea level was 110-130 meters (about 350-400 feet) lower during the ice age, thus exposing large areas of continental shelves.

Figure 3 shows that large changes of sea level are the norm as climate changes. Global sea level, global temperature, and atmospheric greenhouse gas amounts are obviously very highly correlated.

Q. The sea level changes are enormous. Is sea level always changing? What have the consequences been?

A. On millennial time scales resolvable in this graph, sea level, CO2 and global temperature change together. However, close examination shows that sea level has been stable for about the past 7000 years. In that period the planet has been warm enough to prevent an ice sheet from forming on North America, but cool enough for the Greenland and Antarctic ice sheets to be stable. The fact that the Earth cooled slightly over the past 8000 years probably helped to stop further sea level rise.

Sea level stability played a role in the emergence of complex societies. Day et al. (2007) point out that when sea level was rising at the rate of 1 meter per century or faster biological productivity of coastal waters was limited. Thus it is not surprising that when the world’s human population abandoned mobile hunting and gathering in the Neolithic (12,000-7000 years ago) they gathered in small villages in foothills and mountains. Day et al. note that within 1000 years of sea level stabilization, urban (2500 people) societies developed at many places around the 7 world (Figure 4). With the exception of Jericho, on the Jordan River, all of these first urban sites were coastal, where high protein food sources aided development of complex civilizations with class distinctions.

Modern societies have constructed enormous infrastructure on today’s coastlines. More than a billion people live within 25 meter elevation of sea level. This includes practically the entire nation of Bangladesh, almost 300 million Chinese, and large populations in India and Egypt, as well as many historical cities in the developed world, including major European cities, many cities in the Far East, all major East Coast cities in the United States, among hundreds of other cities in the world.

Q. How much will sea level rise if global temperature increases several degrees?

A. Our best guide for the eventual long-term sea level change is the Earth’s history. The last time the Earth was 2-3°C warmer than today, about 3 million years ago, sea level was about 25 meters higher. The last time the planet was 5°C warmer, just prior to the glaciation of Antarctica about 35 million years ago, there were no large ice sheets on the planet. Given today’s ocean basins, if the ice sheets melt entirely, sea level will rise about 70 meters (about 230 feet).

The main uncertainty about future sea level is the rate at which ice sheets melt. This is a “nonlinear” problem in which positive feedbacks allow the possibility of sudden ice sheet collapse and rapid sea level rise. Initial ice sheet response to global warming is necessarily slow, and it is inherently difficult to predict when rapid change would begin. I have argued (Hansen, 2005, 2007a) that a “business-as-usual” growth of greenhouse gases would yield a sea level rise this century of more than a meter, probably several meters, because practically the entire West Antarctic and Greenland ice sheets would be bathed in meltwater during an extended summer melt season.

The Intergovernmental Panel on Climate Change (IPCC, 2007) calculated a sea level rise of only 21-51 cm by 2095 for “business-as-usual” scenarios A2 and A1B, but their calculation included only thermal expansion of the ocean and melting of alpine glaciers, thus omitting the most critical component of sea level change, that from ice sheets. IPCC noted the omission of this component in its sea level projections, because it was unable to reach a consensus on the magnitude of likely ice sheet disintegration. However, much of the media failed to note this caveat in the IPCC report.

Earth’s history reveals many cases when sea level rose several meters per century, in response to forcings much weaker than present human-made climate forcings. Iceberg discharge from Greenland and West Antarctica has recently accelerated. It is difficult to say how fast ice sheet disintegration will proceed, but this issue provides strong incentive for policy makers to slow down the human-made experiment with our planet.

Knowledge of climate sensitivity has improved markedly based on improving paleoclimate data. The information on climate sensitivity, combined with knowledge of how sea level responded to past global warming, has increased concern that we could will to our children a situation in which future sea level change is out of their control.

Q. How can the paleoclimate data reveal the climate sensitivity to forcings?

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