Our Final Hour by Martin Rees

Author:Martin Rees [REES, MARTIN]
Language: eng
Format: epub
Publisher: Basic Books
Published: 2011-11-30T16:00:00+00:00

Earth’s Inconstant Climate

Climatic change has, like extinction of species, characterised Earth throughout its history. But it has, like the extinction rate, been disquietingly speeded up by human actions.

The climate has undergone natural changes on every time scale, from decades to hundreds of millions of years. Even within the era of recorded history the regional climate has varied markedly. It was warmer in Northern Europe a thousand years ago: there were agricultural settlements in Greenland where animals grazed on land that is now ice-covered; and vineyards flourished in England. But there have been prolonged cold periods too. The warm spell seems to have ended by the fifteenth century, to be succeeded by a “little ice age” that continued until the end of the eighteenth century. There are regular records of the ice on the Thames getting so thick during much of that period that fires were lit on it; glaciers in the Alps advanced. The “little ice age” may offer important clues to a question that has been perennially controversial: whether variability of the Sun could trigger alterations in the climate. During this cold spell, the Sun seemed to have been behaving slightly erratically: in the second half of the seventeenth century and the first years of the eighteenth there was a mysterious seventy-year period, now known as the Maunder minimum, after the scientist who first noticed it, in which there were hardly any sunspots at all. The activity on the Sun’s turbulent surface— flares, sunspots and so forth—normally rises to a peak and then drops again, repeating this cycle somewhat unsteadily, but roughly every eleven or twelve years. Claims that this cycle affects the climate date back more than two hundred years, but are still controversial. (It has even been alleged that the economic cycle “tracks” solar activity.) There are also claims that the length of a particular cycle—whether it is closer to eleven years or twelve years—affects the average temperature.

Nobody really understands how sunspots and flaring activity (or their absence) could affect the climate to this extent. Sunspots are linked to the magnetic behaviour of the Sun, and to the flares that generate fast-moving particles that hit Earth. These particles themselves, however, carry only a tiny fraction of the Sun’s energy, but we should be open-minded about the possibility that some “amplifier” in the upper atmosphere might enable them to trigger substantial changes in cloud cover. Scientists have often been caught out in the past, rejecting evidence staring them in the face because they couldn’t at the time think of how to explain it. (A spectacular instance of this is continental drift. The coastline of Europe and Africa seems to fit that of the Americas, like a jigsaw puzzle, as though these landmasses had once been joined and had drifted apart. Until the 1960s, nobody understood how the continents could move, and some leading geophysicists denied the evidence of their own eyes rather than accept that continental motion might be induced by some mechanism that they hadn’t been astute enough to think of.

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