Sun strike

Cycles in solar radiation can’t explain current warming

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As the Victorian enthusiasm for measuring the world and explaining how it worked built up a head of steam, some remarkable characters emerged. One was William Stanley Jevons, who is credited with introducing mathematics to the dark arts of economics. He created an early computer known as the ‘Logic Piano’ and is remembered for Jevons paradox—improving energy effi­ciency reduces energy costs, leading to more energy use rather than less. He also took a stab at linking patterns of boom and bust in the business world with sunspot cycles. His thesis was wrong, but the industrial economy then emerging would go on to influence the effect of solar radiation on the Earth’s climate, becoming even more significant than the natural cycles of the sun itself.

Sunspots are a product of convection currents on the sun’s surface and vary with a cycle of approximately 11 years. The intensity of the sun’s radiation varies by about one tenth of one per cent during this cycle, affecting the weather and climate on Earth. Some commenta­tors have hoped that this would provide sufficient explanation for the warming of the Earth’s atmosphere seen in the past century. However, it is not strong enough to explain all the warming. Moreover, satellite measurements of the sun’s radiation have shown a slight decline in its intensity over the past three cycles. During this same period, the warming of the atmosphere has been the strongest on record—a rise of 0.6ºC in the global average temperature.

Aside from the intensity of the sun’s radia­tion, there is another factor in the Earth-sun relationship influencing climate. Variations in the Earth’s orbit affect the amount of the sun’s radiation reaching the Earth’s surface at different times of the year. The Earth orbits the sun not in a circle but in an ellipse. The extent that the ellipse differs from a circle varies in a cycle that repeats every 100,000 years. Also, the axis of rotation of the Earth, responsible for the seasons, varies a few degrees over a cycle of 40,000 years. Finally, as the Earth rotates, it wobbles in a similar way to a spinning top. This wobble takes around 20,000 years and has the effect of altering which time of the year the Earth is closest to the sun. (Currently, we pass closest to the sun during the southern hemisphere summer, but 11,000 years ago we were closest in the northern hemisphere summer.)

Known as the Milankovitch Cycles, these variations correlate well with the beginning and end of the 20 ice ages that have occurred during the past 2.5 million years. But the change in radiation on its own is not enough to explain the temperature change. The key feed­back mechanism appears to be the intensity of sunlight during the northern hemisphere summer. A much larger proportion of the northern hemisphere is covered in land than in the southern hemisphere. When the northern summer sunlight is most intense, less snow and ice cover the land during winter. Ice acts like a mirror, reflecting sunlight back into space. Once the ice melts, the land absorbs much of the incoming sunlight and is warmed, then warms the air in turn.

Ice sheets have covered large parts of the land in mid-latitudes during most of the past two million years. Currently, we are in a warmer interlude known as an inter-glacial. The warmth peaked about 6000 years ago during what climatologists refer to as the Holocene Maximum—a time when the African monsoon was more intense and widespread, causing a greening of the Sahara Desert. Since then, however, there has been a slow decline in temperature, until the past century. Had the decline continued, the Earth might slowly have headed for another Ice Age.

It’s possible that anthropogenic climate change may actually have saved humanity the horror of contending with a global freeze. But it seems we have rather overdone it.

Past climate can be studied using proxy indicators such as tree rings, sediment cores and isotopes in ice-cores and stalactites. A review paper recently published in Science by Shaun Marcott of Oregon State University and others combined 73 different studies to come up with a graph showing global temperature over the past 11,000 years. They found that the coldest decade was just 200 years ago, and the past decade has been warmer than 75 per cent of the Holocene.

Although the atmosphere has not yet warmed to the temperature of the Holocene Maximum, it will soon exceed it, if warming continues. Superim­posed on the graph is the 3ºC of warming expected by the end of this century if greenhouse gas emissions continue unchecked.

No longer looking like an ice-hockey stick, according to one commentator, the rising graph now more closely resembles the scythe of the Grim Reaper.

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