The lag proves that rising CO₂ did not cause the initial warming as past ice ages ended, but it does not in any way contradict the idea that higher CO₂ levels cause warming.

Sometimes a house gets warmer even when the central heating is turned off. Does this prove that its central heating does not work? Of course not. Perhaps it’s a hot day outside, or the oven’s been left on for hours.

Just as there’s more than one way to heat a house, so there’s more than one way to heat a planet.

Ice cores from Antarctica show that at the end of the recent ice ages, the concentration of carbon dioxide in the atmosphere usually started to rise only after temperatures had begun to climb. There is uncertainty about the timings, partly because the air trapped in the cores is younger than the ice, but it appears the lags might sometimes have been 800 years or more.

Initial warming

This proves that rising CO₂ was not the trigger that caused the initial warming at the end of these ice ages – but no climate scientist has ever made this claim. It certainly does not challenge the idea that more CO₂ heats the planet.

We know that CO₂ is a greenhouse gas because it absorbs and emits certain frequencies of infrared radiation. Basic physics tells us that gases with this property trap heat radiating from the Earth, that the planet would be a lot colder if this effect was not real, and that adding more CO₂ to the atmosphere will trap even more heat.

What is more, CO₂ is just one of several greenhouse gases, and greenhouse gases are just one of many factors affecting the climate. There is no reason to expect a perfect correlation between CO₂ levels and temperature in the past: if there is a big change in another climate “forcing”, the correlation will be obscured.

Orbital variations

So why has Earth regularly switched between ice ages and warmer interglacial periods in the past million years? It has long been thought that this is due to variations in Earth’s orbit, known as Milankovitch cycles. These change the amount and location of solar energy reaching Earth. However, the correlation is not perfect and the heating or cooling effect of these orbital variations is small. It has also long been recognized that they cannot fully explain the dramatic temperature switches between ice ages and interglacials.

So if orbital changes did cause the recent ice ages to come and go, there must also have been some kind of feedback effect that amplified the changes in temperatures they produced. Ice is one contender: as the great ice sheets that covered large areas of the planet during the ice ages melted, less of the Sun’s energy would have been reflected back into space, accelerating the warming. But the melting of ice lags behind the beginning of interglacial periods by far more than the rises in CO₂.

Another feedback contender, suggested over a century ago, is CO₂. In the past decade, detailed studies of ice cores have shown there is a remarkable correlation between CO₂ levels and temperature over the past half a million years (see Vostok ice cores show constant CO₂ as temperatures fell).

Rising together

It takes about 5000 years for an ice age to end and, after the initial 800-year lag, temperature and CO₂ concentrations in the atmosphere rise together for a further 4200 years.

What seems to have happened at the end of the recent ice ages is that some factor – most probably orbital changes – caused a rise in temperature. This led to an increase in CO₂, resulting in further warming that caused more CO₂ to be released and so on the positive feedback that amplified a small change in temperature. At some point, the shrinking of the ice sheets further amplified the warming.

Models suggest that rising greenhouse gases, including CO₂, explain about 40% of the warming as the ice ages ended. The figure is uncertain because it depends on how the extent of ice coverage changed over time, and there is no way to pin this down precisely.

Biological activity

The source of this extra carbon was the oceans, but why did they release CO₂ as the planet began to warm? Many factors played a role and the details are still far from clear.

CO₂ is less soluble in warmer water, but its release as a result of warming seawater can explain only part of the increase in CO₂. And the reduction in salinity as the ice melted would have partly counteracted this effect.

A reduction in biological activity may have played a bigger role. Tropical oceans tend to release CO₂, while cooler seas soak up CO₂ from the atmosphere as phytoplankton grow and fall to the ocean floor. Changes in factors such as winds, ice cover, and salinity would have cut productivity, leading to a rise in CO₂.

Runaway Prevention

The ice ages show that temperature can determine CO₂ as well as CO₂ driving temperature. Some skeptics – not scientists – have seized upon this idea and are claiming that the relation is one way, that temperature determines CO₂ levels but CO₂ levels do not affect temperature.

To repeat, the evidence that CO₂ is a greenhouse gas depends mainly on physics, not on the correlation with past temperature, which tells us nothing about cause and effect. And while the rises in CO₂ a few hundred years after the start of interglacials can only be explained by rising temperatures, the full extent of the temperature increases over the following 4000 years can only be explained by the rise in CO₂ levels.

What is more, further back in past there are examples of warming triggered by rises in greenhouse gases, such as the Palaeo-Eocene Thermal Maximum 55 million years ago (see Climate myths: It’s been far warmer in the past, what’s the big deal?).

Finally, if higher temperatures lead to more CO₂ and more CO₂ leads to higher temperatures, why doesn’t this positive feedback lead to a runaway greenhouse effect? There are various limiting factors that kick in, the most important being that infrared radiation emitted by Earth increases exponentially with temperature, so as long as some infrared can escape from the atmosphere, at some point heat loss catches up with heat retention.

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