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After a two-month hiatus, I’ve finally found the time to pick up the thread on doubts about global warming and humans’ role in it. In previous installments, people wrote in with the reasons why they were skeptical and I tried to synthesize the responses. In a rough-and-ready poll to gauge which line of reasoning resonated the most, the favorite appeared to be #2: The present warming could be a natural uptick.
There’s a lot to be said on this topic, so I’ll start by biting off a piece of it. What I’d like to try to do is to get beyond the he said/she said debate that it’s all too easy to fall into, and see whether a fresh look at the data can break the logjam.
The Average Argument
The standard argument for anthropogenic warming goes something like this:
- Surface temperature readings jiggle up and down, but over the course of the past century, their overall trend is up.
- The levels of carbon dioxide and certain other gases in the atmosphere have also been rising. The concentration of CO2 in seawater has increased as well. The total observed increase is consistent with the amount of fuel humanity has burned and the amount of land it has cleared since the industrial revolution began.
- Basic theory links these two trends. Because of the gases, a smaller fraction of the thermal radiation emitted by the surface escapes into space. The planet becomes a net absorber of energy. The surface heats up and emits more thermal radiation, until the total amount of escaping thermal radiation again balances the energy deposited by sunlight.
- Natural factors, such as variations in the sun’s output, have been too small to account for the observed temperature increase.
To put this argument into a quantitative framework, climatologists estimate how much extra heating the addition of these gases has caused per unit area of Earth’s surface. Other influences on the climate — aerosol (particulate) emissions, orbital oscillations, solar variability — can be measured in the same way. All these influences, or "forcings", can then be treated interchangeably and added together. Subjected to this net total forcing, the climate adjusts — shifting winds, clouds, precipitation, and other weather patterns, which can either amplify or offset the forcing — and the global average temperature reaches some new value.
Filling in some very rough numbers, the current forcing is 2 watts per square meter, mostly from carbon dioxide, methane, and low-altitude ozone, minus the cooling wrought by particulates. Of this forcing, the oceans appear to have absorbed half, leaving the atmosphere to take up the rest. According to the geologic record, 1 W/sq m should lead to about half a degree Celsius of warming — which matches the observed increase. This temperature rise is greater than you’d expect from the energy input provided by the forcing, so the climate system appears to be overreacting; feedback loops are amplifying the direct effect.
In this framework, skeptical arguments #2 and #3 have two prongs: first, that the forcings haven’t been properly enumerated; and second, that the climate machine does not respond in the predicted way. Respondents to my original posting put forward various natural forcings that they said might outweigh the anthropogenic ones. Others suggested that the response to forcings gets lost in the noise of intrinsic variability.
Elsewhere, I’ll examine these hypotheses, but my goal here is to get away from the common-currency approach altogether. If all the forcings are treated interchangeably, it becomes hard to verify whether the 2 watts per square meter is due to this or that influence, whether a crucial forcing has gone overlooked altogether, or whether the half-degree of warming is due to a large forcing on an unresponsive climate, a small forcing on a hair-trigger one, or the coincidental wiggling of a climate wont to do its own thing.
The above argument uses, in essence, two data points: the average annual temperatures at the beginning of the century and today. These barely scrape the surface of the available data. Climatologists have maps and time series showing how a boatload of climate variables — mean temperature, temperature ranges, air pressure, precipitation, and so on — vary in time and in space, horizontally across the surface and vertically through the atmosphere. These data sets are a gold mine for resolving ambiguity, because the different forcings leave distinct fingerprints.
For instance, carbon dioxide spreads out evenly, whereas aerosols tend to remain concentrated above polluting regions. Carbon dioxide emissions have been steadily climbing over time, whereas sulfur dioxide emissions, the largest contributor to aerosol loading, recently peaked. Such patterns make it possible to tease out their relative contributions. In fact, when scientists attribute xpercent of warming to greenhouse gases, y percent to solar fluctuations, and so on, they derive these numbers by mixing and matching the various fingerprints to reproduce the observed trends.
One of the handiest things about fingerprints is that they help to make sense of climate models. One of the things that troubles many people, skeptics and otherwise, about models is that they are black boxes whose inner workings are obscure and whose output has to be taken on trust. Although quantifying the fingerprints still requires running a climate model, the response can be understood in terms of basic principles. Wall Street Journal science writer Sharon Begley had a nice article in May on this topic, and below I’ll review some of the specific fingerprints.
On the downside, different forcings sometimes have similar fingerprints. Even when they don’t, the feedbacks, inertia, and other internal dynamics of the climate system can mute the differences. So using fingerprints involves uncertainties, which take some effort to quantify.
The study of fingerprints got going in the 1980s and came into its own in the mid-1990s, when Benjamin Santer of Lawrence Livermore and his colleagues made the first rigorous argument for an anthropogenic contribution to surface temperatures. The IPCC devoted a chapter to the subject in its 2001 report and is preparing an even more extensive discussion for its next major report, due out next year. Over the years, researchers have considered ever more variables besides temperature and ever more forcings besides greenhouse gases. They have merged spatial and temporal patterns, looked at regional as well as global scales, and developed more sophisticated mathematical tools.
A major breakthrough occurred last year when researchers figured out why temperature measurements at the surface were not matching those higher up in the troposphere, the lowest layer of the atmosphere — a discrepancy that was one of the most powerful points made by climate skeptics. One factor turned out to be a slow orbital decay of the satellites taking the atmospheric measurements, which produced a spurious cooling that masked the warming trend. In May, the two scientists who had identified the mismatch — John Christy and Roy Spencer of the University of Alabama, Huntsville — were among the co-authors of a report that laid the problem to rest. Some discrepancy lingers, but the report argued that other observational biases are probably responsible.
One by One
Here are some of the fingerprints. For those people who, like me, hunger to grasp the basic physics behind the summary, I elaborate in an appendix below.
- Greenhouse gases. Carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons warm the surface and troposphere (the lowest atmospheric layer) and cool the stratosphere (the next layer up). These gases get mixed in quickly, so their effects are nearly symmetrical between the Northern and Southern Hemispheres. At the surface, the warming is greater at night than during the day, during the winter than during the summer, and at high latitudes than at low latitudes. Greenhouse gas forcing has grown slowly and steadily over the industrial era.
- Aerosols. Sulfate aerosols cool the surface, troposphere, and stratosphere alike. At the surface, the temporal pattern is exactly the opposite that of greenhouse gases: cooling is greater at day than during the night, during the summer than during the winter, and at low latitudes than at high latitudes. Black carbon (soot) warms whichever layer of the atmosphere it is found in. Aerosols exert their strongest effect over the regions where they are emitted, which are mostly in the Northern Hemisphere. The sulfate forcing grew more rapidly than the greenhouse one from 1945 to 1980 and has since been declining.
- Ozone. The depletion of ozone from the ozone layer cools the stratosphere; the cooling is greatest at high latitudes, where the layer has suffered the most. The depletion also causes a small amount of cooling at the surface and in the lower troposphere. As the phase-out of ozone-destroying chemicals allows the layer to rebuild itself, the stratosphere should rewarm. Ozone near the ground (a component of smog) warms the surface and lower troposphere.
- Solar variability. An increase in the intensity of the sun warms the troposphere and stratosphere nearly equally.
- Volcanic eruptions. By hurling dust to high altitudes, explosive volcanoes cool the troposphere and warm the stratosphere. The stratospheric warming lasts one to two years, and the surface and troposphere cooling three to five years. Volcanoes also reduce the global average precipitation.
- Generic effects. Some fingerprints are not specific to one forcing or another; they occur when the temperature changes for whatever reason. These fingerprints are useful for cross-checking the data. For instance, any temperature changes should be greater for land than for sea, and because the Northern Hemisphere has more land than the Southern, any trends should be more pronounced there. In the tropics, temperature changes should be greater in the troposphere than at the surface, because moist air releases heat as it rises and condenses. Also, if the trends are short-term (driven mainly by, say, volcanic eruptions or the 11-year solar cycle), they should have little effect on temperatures in the ocean depths or in boreholes. It takes a while for heat to penetrate down, so a subsurface temperature profile provides a measure of cumulative changes, whereas the surface and atmosphere mark the present moment, with all its vicissitudes.
- Internal (unforced) climate variability. By definition, a forcing adds energy to the climate system, whereas natural variability merely redistributes it. So if the recent string of hot years is just a wiggle, then the total heat content of the climate system — all the oceans plus the ice caps plus the atmosphere — should be constant. Another difference is that a forcing shifts the range of climate events, whereas variability is the range. If historical data indicate that heat waves have become more common and cold snaps less so, the range itself must have shifted, indicating a forcing rather than variability.
View from the Crime Scene
So what do the data say?
- Vertical patterns. Since 1979, when continuous satellites observations began, the surface and troposphere have warmed and the stratosphere has cooled. Consequently, the boundary between the troposphere and stratosphere has risen by about 170 meters over that period; it has risen more at high latitudes than at low ones. All this is exactly what you’d expect from greenhouse gases, with some limited offsetting by sulfate aerosol cooling. The stratosphere over Antarctica is especially cool, apparently because of the ozone hole. d03;
- Horizontal patterns. Pretty much the entire surface has gotten warmer, high latitudes more than lower ones. In the early 20th century, land regions and oceans warmed at similar rates, but since the mid-1970s, the land has sped ahead. On the whole, the Northern Hemisphere has warmed more than the Southern. Again, this is just what greenhouse gases would do. d03;
- Temporal patterns. Nighttime and wintertime temperatures have increased more than daytime and summertime ones, compressing the diurnal and seasonal temperature ranges over land worldwide. The trends are apparent even on a regional level, including North America, albeit with lower statistical significance. There are fewer frost days, fewer frigid extremes, and more warm nights. Greenhouse gases would produce these very effects. d03;
- Energy balance. An interesting paper last year looked at vertical ocean temperature profiles since 1960 using new data sets and found widespread heating. The upper 700 meters of Atlantic waters and upper 100 meters of the Pacific and Indian Ocean are warmer than the deeper layers. The different depths are exactly what you’d expect, because the Atlantic has stronger convection currents than the other oceans do. All the oceans have warmed; there isn’t the zero-sum game of warming and cooling you’d expect from natural variability. All this is consistent with greenhouse gases. Per unit area, the northern seas have warmed less than the southern ones, which makes sense if greenhouse gases have caused an overall warming trend, offset by sulfate aerosols in the northern climes where they are concentrated. d03;
When climatologists run the fingerprinting analysis for different historical epochs, they find that temperature fluctuations prior to the Industrial Revolution were driven primarily by solar and volcanic forcings. In the early 20th century, natural and anthropogenic forcings seem to contribute equally. From midcentury onwards, greenhouse gases rule.
So unless I’m missing something, it seems to me that the case for anthropogenic warming is pretty strong. Is it unequivocal? Well, nothing in life ever is. There might be some low-frequency natural forcing. Plenty of i‘s in fingerprinting analyses remain to be dotted. Based on the knowledge we have so far, however, I have to call ‘em as I see ‘em.