Sat, Jun 19, 2010 page9

• The irrefutable physics of global warming

  The laws of physics do not surrender to opposition. For the past 35 years global warming has unfolded as predicted by science

  • By Stefan Rahmstorf
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  According to the worldwide temperature records maintained by NASA and the US National Oceanic and Atmospheric Administration (NOAA), this April was the hottest April globally, for at least 130 years. The past twelve months have been the hottest 12-month period since measurements began.

  That is what the data from weather stations and ships shows. But if you prefer satellite data, the picture is similar. Satellite data shows this March to be the hottest March on record, with April ranking second-hottest; whereas surface data has it the other way round, with March the second-hottest and April the hottest.

  Scientifically speaking, long-term trends are much more important. For the past 30 years — that’s how long satellite measurements have been taken — the trend is clearly upward and similar in magnitude in all the available data sets.

  

  Should you still have doubts that the planet is heating up, look at the shrinking mountain glaciers around the world or the declining sea-ice cover on the Arctic Ocean, which in recent summers has been little more than half its size in the 1970s.

  What is causing this climatic warming? Physics tells us: if you want to know why it is getting warmer, find the heat source (the first law of thermodynamics states that energy is always conserved). We thus have to look at the heat balance of our planet to understand the reason for the warming.

  That is surprisingly simple: there is only one source of heat coming in, and that is radiation from the sun (which is largely visible light or what physicists call short-wave radiation) and there is only one form of heat leaving the planet, and that is radiative heat (which is invisible or what physicists call long-wave radiation). They are essentially the same physical phenomenon, the difference in wavelength occuring because the sun is so much hotter than Earth.

  So, could changes in solar radiation explain the warming of the planet? Measurements of incoming solar radiation show that it has not increased in the past 50 years. In fact, the record even shows a small decrease. But the record’s predominant feature is the recurrence of solar radiation cycles lasting about 11 years (called Schwabe cycles, after the astronomer who discovered them in 1843).

  Over the past few years, we have been in the deepest and longest minimum part of the Schwabe cycle since satellite measurements began. In other words, while global temperatures are at a record high, the sun has been at its dimmest in decades. Changes in solar activity clearly cannot explain global warming.

  But that leaves another factor affecting incoming solar radiation: how much gets reflected back into space by ice, snow, clouds, desert sand, and other bright, mirror-like surfaces. Indeed, a part of the observed warming is due to less reflection, as snow and ice cover has shrunk. This allows more solar heat to be taken up in the climate system, which is one reason why the Arctic has warmed at a faster rate than other parts of the world.

  But shrinking snow and ice cover is itself a result of warming, so reduced reflection of solar rays is not the primary cause of warming. Rather, it is a feedback that amplifies warming.

  Humans have altered the brightness of the Earth — as seen from space — in more direct ways. But converting forests to farmland (which is brighter than forested areas) and adding smog particles to the atmosphere (which reflect sunlight) have increased the reflection of solar radiation, thus tending to offset some of the global warming that would otherwise have occurred.

  So we are left with the second part of the planetary heat budget: radiative heat escaping to space. That can be changed by adding heat-trapping gases to the atmosphere — the so-called greenhouse gases, which absorb long-wave radiation on its way out and send some of it back towards the surface.

  The importance of this “greenhouse effect” has been known in science since the nineteenth century, when Joseph Fourier coined the term. Perhaps nobody has described it more succinctly than the British physicist John Tyndall, who was the first to measure the effect in his laboratory in 1859 for a number of gases, including carbon dioxide. He wrote: “The atmosphere admits of the entrance of solar heat, but checks its exit; and the result is a tendency to accumulate heat at the surface of the planet.”

  We know from measurements that greenhouse gases are accumulating in the Earth’s atmosphere. Carbon-dioxide levels are one-third higher now than at any time in the past million years, owing to industrial emissions. We can calculate how much this has changed the Earth’s heat balance — and it is just the right amount to explain the observed warming. That is one of several reasons why hardly any serious climate scientist doubts that greenhouse gases are the cause of global warming.

  In fact, this warming was predicted before it was observed. The rise in carbon dioxide levels has been known since the 1960s. In 1975, the American climatologist Wallace Broecker published a paper in the journal Science, entitled “Are We on the Brink of a Pronounced Global Warming?” This correctly predicted “that the present cooling trend will, within a decade or so, give way to a pronounced warming induced by carbon dioxide” and that “by early in the next century [carbon dioxide] will have driven the mean planetary temperature beyond the limits experienced during the last 1,000 years.” He predicted overall global warming in the 20th century of 0.8ºC. He was right on all counts.

  Many are lining up to oppose the science of global warming. But the laws of physics don’t surrender to opposition. For the past 35 years, global warming has unfolded as predicted by science and it will most likely continue to do so until we stop it by cutting carbon dioxide emissions.

  Stefan Rahmstorf is a professor of Physics of the Oceans at Potsdam University and department head at the Potsdam Institute for Climate Impact. Research.

  Copyright: Project Syndicate
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