Storms of My Grandchildren (11 page)

If you look at the sun through a polarized lens, which transmits light vibrating in one preferred direction, and rotate the lens, the intensity of radiation does not change. The light is unpolarized because the thermal emission, from the sun in this case, includes many packets of radiation vibrating in random directions in the plane perpendicular to the propagation direction.

However, when sunlight is reflected from a surface or scattered by a particle, it can become polarized. That is the reason for wearing polarizing sunglasses—reflection from a water or road surface is polarized. The water or road reflects mostly the radiation packets vibrating horizontally. So the polarized lenses in sunglasses are oriented vertically to cut down the glare.

The polarization of light scattered by small particles—aerosols or cloud drops—contains an enormous amount of information on the nature of the scattering particles, if the polarization is measured to high accuracy. In 1969 I studied as a National Science Foundation fellow at the University of Leiden Observatory in the Netherlands, under the world’s leading expert on light scattering, Professor Henk van de Hulst, and worked also with his top protégé, Joop Hovenier. We showed that the polarization reveals the aerosol amount, the size and shape of the aerosols, and even their index of refraction. This latter quantity—a measure of the angle at which a light ray is bent when it enters the particle—helps to identify aerosol composition.

A basic understanding of the information about reflected solar and emitted thermal radiation illustrated in figure 5 tells us what we need in order to determine the aerosol climate forcing. The required observations are (1) polarization of reflected sunlight to an accuracy of about a tenth of a percent, with a given spot on the ground looked at from several different directions as the satellite passes overhead, and with the measurements made at several wavelengths spread over the solar spectrum; and (2) infrared emission measured with a high-precision interferometer, that is, with an instrument that gives the best wavelength-to-wavelength precision.

Okay, so in 1970 the physics was already clear. The information content on aerosols and their effect on clouds required polarimetry of reflected sunlight and interferometry of thermal emission. But measurements were not started then—climate change was not an urgent issue in 1970. My scientific interest at that time was with other planets.

Almost twenty years later, climate was an issue. In December 1989 I received a letter from senators Al Gore and Barbara Mikulski inviting me to a “roundtable” meeting in Gore’s office. They wanted to discuss three proposed programs, all seeking support from U.S. taxpayers: the NASA Earth Observing System, the U.S. Global Change Research Program, and the International Geosphere-Biosphere Program.

I was asked to participate as a scientist, not as a representative of NASA. None of the other participants (James Baker, Francis Bretherton, Tom Lovejoy, Gordon MacDonald, Mike McElroy, Irving Mintzer, Bill Moomaw, and George Woodwell) were government employees. It was not unusual for me to be the only government employee in science advisory meetings—I long had a reputation for giving frank scientific opinions, without concern for institutional implications.

The day of the meeting happened to be the coldest day in the eastern United States in several years. The heating system in the Russell Senate Office Building faltered that day, and it was freezing in the meeting room. As I walked in, Al Gore said, “Say, aren’t you the guy who…” cutting off his sentence at that point. Everybody had a good laugh. Gore put on his jacket and instructed his staff to bring in pots of hot coffee.

Al Gore was remarkable. He asked questions around the table about the major scientific issues in earth sciences. He was also the note-taker. Every now and then he would say, “Okay, here is what I understand,” repeating the essence in language that an educated person could understand. He did a better job than most scientists could do—certainly better than I could do.

A second meeting occurred in Senator Gore’s office in January 1990. The invitation letter included a list of twenty-three scientific topics that came out of the first meeting. After seeing that long list, I was determined to raise a fundamental matter at the second meeting: the need for a scientifically defined focus for an observing program.

There is a valid scientific rationale for putting large instruments in space to observe climate and other processes on Earth in fine detail. But, in addition, there are highly precise measurements needed to understand long-term climate change that must be continued for decades. For example, human-made sources of aerosols change slowly over the years. The aerosol changes need to be measured, as well as the effect of aerosol changes on clouds. But clouds change for other reasons too—perhaps in concert with solar irradiance changes. So measurements must be continued over at least a couple of ten-to-twelve-year solar cycles.

I went to the second Gore-Mikulski meeting with a table summarizing the measurements needed to analyze long-term climate change (see the table in appendix 2 on page 281). Climate forcings that must be precisely monitored are solar irradiance, greenhouse gases, aerosols, and surface properties. Climate feedbacks are clouds, water vapor, and surface ice and snow. Some of the quantities are both forcing and feedback, but with precise global measurements over sufficient time, it is possible to sort that out.

One conclusion was that all essential measurements could be made by four instruments, each moderate in size and cost. Two of the instruments, the polarimeter, measuring reflected sunlight, and interferometer, measuring thermal emission, would need to be on the same satellite, doing their measurements more or less simultaneously while looking at the same location. A third instrument would precisely monitor the sun’s irradiance. A fourth instrument would make precise measurements of aerosols and gases in tenuous higher layers of Earth’s atmosphere by observing the sun from a satellite through Earth’s atmosphere at sunrise and sunset.

I knew that getting support for taking these measurements would not be easy. Before the second Gore-Mikulski roundtable meeting, Michael McElroy, chairman of earth and planetary sciences at Harvard University, pulled me aside. McElroy related that he had recently met the head of the Earth Observing System program, Shelby Tilford, who seemed to be aware of the criticisms of that program expressed at the first Gore-Mikulski meeting. McElroy asked him, “Did you have a mole at the meeting?” and received a flushed response: “Watch it, McElroy, or you will end up in the same box as Hansen.”

It seemed to me that the communication problem could be overcome. Almost every scientist told me privately that I was right about the need to obtain this precise long-term data from small satellites. So for several years I continued advocating this concept to measure the key climate forcings and feedbacks. In 1992, with colleagues Bill Rossow and Inez Fung, I published a comprehensive workshop report that described the science rationale. The workshop included the participation and support of a large number of the best relevant scientists in the country.

Although those were the explanations I was often told, my opinion, based on many years of observing the process, is that there was another, more powerful reason behind the scenes. I refer to the special interests—the individuals, organizations, and industries that obtained support from the program focused on large satellite systems. Whatever the reasons, the result is that, twenty years later, we still do not know the aerosol climate forcing or how it is changing.

These same arguments come up as a rationale for ignoring policies proposed to alleviate climate change (as I will show in later chapters). This is happening in discussions both within the United States and in the United Nations, where the objective is to find an effective treaty to succeed the Kyoto Protocol.

In both the United States and the United Nations, the real forces at work have little to do with perfect versus good or trains having already left the station. Indeed, those arguments are readily shown to be bogus. But the story is not a simple one with easily identified good guys and bad guys. For example, someone with the noblest of objectives may feel that he has found a way to work the post-Kyoto system so as to benefit his specific noble objective. In that case, he may be willing to support a system that has no chance of stabilizing climate, perhaps thinking we can improve the system later.

I do not care much whether you try to understand polarimeters or interferometers. But I care a lot whether you understand policy discussions that are going on in Washington and other capitals around the world. If we let special interests rule, my grandchildren and yours will pay the price.

H
UMANITY TREADS TODAY ON A SLIPPERY slope. As we continue to pump greenhouse gases into the air, we move onto a steeper, even more slippery incline. We seem oblivious to the danger—unaware how close we may be to a situation in which a catastrophic slip becomes practically unavoidable, a slip where we suddenly lose all control and are pulled into a torrential stream that hurls us over a precipice to our demise.

You may say, “Surely you are joking, Mr. Hansen!” Would that I were. Human-made climate change is, indeed, the greatest threat civilization faces. Skepticism at such an extreme statement is understandable. The number of degrees Celsius involved in global warming seems small compared with day-to-day temperature fluctuations. How can the warming of the past century, about 0.8 degree Celsius (about 1.5 degrees Fahrenheit), be so important? Even if the warming increases to several degrees this century, as will occur if we continue business-as-usual increases in fossil fuel use, how can warming of several degrees destroy civilization?

The paradox of global warming, the fact that mild heating can have dramatic consequences, first occurred to me one summer day in 1976 as Anniek and I were driving home with our son, Erik, after spending the afternoon at Jones Beach on Long Island. When we had arrived at the beach near midday, we needed to find a spot near the water to avoid the scorching hot sand. Yet by late afternoon it became very cool as a strong wind from the ocean whipped up whitecaps. Erik and I had goose bumps as we ran along the foamy shoreline and watched the churning waves.

Earlier that year Andy Lacis and I, and three colleagues, had calculated the climate forcing by all human-made greenhouse gases. Their heating of Earth’s surface had reached a level of almost 2 watts per square meter. The paradox to me as we ran along the beach was the contrast between nature’s awesome forces and this seemingly feeble heating. It was hard for me to see how the warmth of two tiny 1-watt bulbs over each square meter could command the wind and waves or smooth our goose bumps. And wouldn’t any such low-wattage heating of the ocean surface be quickly dissipated to great depths?

Ah, but remember, climate change between the present interglacial period and the ice age 20,000 years ago was maintained by a forcing of only about 6.5 watts—yet that forcing produced a different world, with Canada and parts of the United States under a thick ice sheet, and a sea level 350 feet lower than it is today. Moreover, the forcings composing the 6.5 watts—3.5 watts from a change of surface reflectivity and 3 watts from a change of atmospheric gases—were, in fact, slow climate feedbacks instigated by a far weaker forcing that was much less than 1 watt on global annual average: the perturbations of insolation on Earth’s surface due to small changes in Earth’s orbit.

On the contrary. Human-made climate forcing, by paleoclimate standards, is large and changes in decades, not tens of thousands of years. It is important to determine the response time to this forcing via analysis and understanding of the climate system. Unfortunately, paleoclimate provides no known empirical data on the response time for a large, rapid, positive (warming) forcing that perseveres. Volcanic eruptions and asteroid impacts cause rapid and large climate forcings, but those forcings are negative (cooling) and brief. The saw-toothed climate response (shown in figure 3 on page 37) to symmetric orbital forcings provides a hint, however. Warmings proceed more rapidly than coolings, presumably because the growth of an ice sheet is limited by the rate of snowfall in a cold place, while multiple amplifying feedbacks can speed the wet process of ice sheet disintegration, once it begins in earnest.

I realized early in this decade that there was a growing danger of pushing the climate system to a point such that future disasters might occur out of our control. The concepts are not difficult. A look at just two phenomena, inertia and feedbacks, is enough to yield the conclusion: We really do have a planet in peril.

Three big sources of inertia affect global warming and its consequences: the ocean, the ice sheets, and world energy systems. The ocean is, on average, about two and a half miles deep. It takes the ocean a long time—centuries—to fully warm up in response to human-made greenhouse gases. So even if we stabilize atmospheric composition at today’s levels, the planet will still continue to heat up, because the ocean will continue to warm. If the ocean were the only source of inertia, additional warming over the course of this century—with no additional gases—would be a few tenths of a degree Celsius.

The nature of the second source of inertia, ice sheet inertia, is—in one key sense—almost opposite that of the ocean. Despite the dynamical tricks the ocean can play, its thermal inertia effect is pretty straightforward. Ocean surface temperature, the quantity that most affects global climate, achieves half of its equilibrium (long-term) response to a forcing within a few decades. Yes, it takes many more decades, even centuries, for the full response, but the ocean has already achieved about half or more of its full response to greenhouse gases added to the air in the past century.

Ice sheet response to global warming is quite the contrary. Ice sheet size changes little at first, and thus sea level changes only slowly. As the planet gets warmer, the area on the ice sheet with summer melt increases. And as the ocean warms, ice “shelves”—tongues of the ice sheet that reach out into the ocean and are grounded on the ocean floor—also begin to melt. As ice shelves disappear and the ice sheet is “softened up” by surface warming and meltwater, movement of ice and discharge of giant icebergs via ice “streams” become more rapid, leading to the possibility that large portions of the ice sheet will collapse.

If we continue burning fossil fuels at current rates, ice sheet collapse and sea level rise of at least several meters is a dead certainty. We know this from paleoclimate records showing how large the ice sheets were as a function of global temperature. The only question is how fast ice sheet disintegration will occur.

Once ice sheets begin to collapse, sea level can rise rapidly. For example, about 14,000 years ago, as Earth emerged from the last ice age and became warmer, sea level rose at an average rate of 1 meter every 20 or 25 years, a rate that continued for several centuries. The danger today is that we may allow ocean warming and “softening up” of ice sheets to reach a point such that the dynamical process of collapse takes over. And then it would be too late—we cannot tie a rope or build a wall around a mile-thick ice sheet.

The third source of inertia is our fossil-fuel-based energy system. The transitions from wood to coal to oil to gas each required several decades—and recently, as oil and gas supplies tightened, we have begun moving back toward more coal use. Indeed, coal is again the largest source of carbon dioxide emissions.

The upshot regarding energy system inertia is this: Humanity today is heavily dependent on fossil fuels—coal, oil, and gas—for most of our energy. When we realize that it is necessary to phase out fossil fuels, that transition will not be quick—it will take at least several decades to replace our enormous fossil fuel infrastructure. In the meantime more greenhouse gas emissions and more climate change will be occurring.

Climate feedbacks interact with inertia. Feedbacks (as discussed in chapter 3) are responses to climate change that can either amplify or diminish the climate change. There is no inherent reason for our climate to be dominated by amplifying feedbacks. Indeed, on very long time scales important diminishing feedbacks come into play (see chapters 8 and 10).

However, it turns out that amplifying feedbacks are dominant on time scales from decades to hundreds of thousands of years. Water (including water vapor, ice, and snow) plays a big role. A colder planet has a brighter surface and absorbs less sunlight, mainly because of the high reflectivity of ice and snow surfaces. A warmer planet has more greenhouse gases in the air, especially water vapor, as well as darker vegetated land areas. Dominance of these two amplifying feedbacks, the planet’s surface reflectivity and the amount of greenhouse gases in the air, is the reason climate whipsawed between glacial and interglacial states in response to small insolation changes caused by slight perturbations of Earth’s orbit.

Amplifying feedbacks that were expected to occur only slowly have begun to come into play in the past few years. These feedbacks include significant reduction in ice sheets, release of greenhouse gases from melting permafrost and Arctic continental shelves, and movement of climatic zones with resulting changes in vegetation distributions. These feedbacks were not incorporated in most climate simulations, such as those of the Intergovernmental Panel on Climate Change (IPCC). Yet these “slow” feedbacks are already beginning to emerge in the real world.

Rats! That is a problem. Climate inertia causes more warming to be in the pipeline. Feedbacks will amplify that warming. So “inertia” was a Trojan horse—it only seemed like a friend. It lulled us to sleep, and we did not see what was happening. Now we have a situation with big impacts on the horizon—possibly including ice sheet collapse, ecosystem collapse, and species extinction, the dangers of which I will discuss later.

What to do? If we run around as if our hair is on fire, flapping our arms, people will not take us seriously. Besides, we are not in a hopeless situation. Rational, feasible actions could avert disastrous consequences, if the actions are prompt and strategic. Feedbacks work in both directions—if a forcing is negative, amplifying feedbacks will increase the cooling effect.

If we wish to stabilize Earth’s climate, we do not need to return its atmospheric composition to preindustrial levels. What we must do, to first order, is reduce the planet’s energy imbalance to near zero. Of course, the climate then would be stabilized at its current state, not at its preindustrial state. Climate may need to be a tad cooler than today, if, for example, we want ice sheets to be stable. That may require a slight additional adjustment of the human-made climate forcing. But let’s not get ahead of the story.

First, I wanted to make clear the danger that business-as-usual emissions would lead to eventual ice sheet disintegration and large sea level rise. Second, in contrast to a 2001 IPCC report implying that global warming of about 3 degrees Celsius (above the 1990 level) was needed to reach the dangerous level, it seemed clear to me that 3-degree global warming, or even 2-degree, was a recipe for global disaster. Third, scientists needed to define scenarios that would keep global warming within tolerable limits. Otherwise we aid and abet government energy departments that seem to be working hand in glove with the fossil fuel industry, accepting as a god-given fact that humanity will proceed to burn all fossil fuels.

I prefer to start with paleoclimate, the lessons of history, which provide our best measure of how Earth responds to changing boundary conditions or forcings. Second, as a measure of how rapidly climate can change, we need to look at what is happening now—observations of the ongoing climate response to fast-changing human and natural forcings. Climate models come third. Models aid interpretation of past climate, and they are needed to project future changes. So models are valuable, but only when used with knowledge of their capabilities and limitations.