The Climate in Emergency

A weekly blog on science, news, and ideas related to climate change

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How Quickly Can We Cool?

I could write about lots of horrible things going on in the news this week. Unfortunately, I suspect there will be plenty of horrible news item to write about next week. This week I want to write about global cooling instead.

I’m working on a book set after the end of the age of fossil fuel, which means I need to understand how the climate responds to a falling carbon dioxide level. Obviously, average temperatures would fall, but how quickly? Warming has a lag time of several decades, because it takes time for heat to build up. Logically, cooling should be much faster. In bed, add an extra blanket and you won’t warm up for a few minutes, but kick your blankets off and you’ll cool down right away. But faster and instant are not the same thing, so how long would global cooling take? Since I need to read up on the issue anyway, I figured I’d share my results with you.

My fictional scenario is that a pandemic triggers the end of civilization, the total end of fossil fuel use, and a 90% reduction of the human population. It’s a complex and complicated scenario, because while most carbon dioxide emissions end, some types of methane emissions, such as leaking well-heads or outgassing landfills, would continue or even increase–and methane is a more powerful greenhouse gas than CO2 is. Would a net increase or decrease in climate-forcing power result? A smaller human population would allow widespread reforestation, but the warming that has already occurred would continue to cause forest dieback in some areas. Would there be a net increase or decrease in forest biomass?

Also, the planet would continue adjusting to the greenhouse gas and the heat that is already present. If greenhouse gas levels stabilized where they are now, temperatures would continue to rise for several decades. And even if the planetary temperature stabilized where it is now anyway, glaciers and permafrost would continue to melt. Melting permafrost, remember, releases methane, so the greenhouse gas concentration might continue to rise. Potential feedback loops abound.

I would love to stick all these variables into some giant computer and run a full simulation, but I don’t have that option. The best I can reasonably hope for is a definitive answer to just one question; assuming the greenhouse gas levels do fall, how long until temperatures start falling also?

Unfortunately, since the chance of my scenario occurring any time soon is very small, nobody seems to be studying what a falling greenhouse gas level would look like.

Fortunately, a version of my scenario did happen about five hundred years ago, when diseases killed off 90% of the population of the Americas, allowing widespread reforestation and causing the second, deeper phase of the Little Ice Age. So, how fast did that happen?

According to one estimate, the reforestation of the Americas could have removed anywhere from two to 17 billion tons of carbon dioxide from the atmosphere. That’s somewhere between 10 and 50% of the CO2 reduction recorded in ice core samples from Antarctica, so something else was going on also. There are various possibilities. But carbon dioxide levels do tend to track known European and Asian pandemics, which also allowed reforestation. The first, less severe phase of the Little Ice Age, may have been, in part, related to reforestation after the Black Death.

So, let’s look at the timeline–since researchers at Stanford University must think the timing of the second phase of the cold period is consistent with it being influenced by the American reforestation. Does the timeline suggest a lag exists?

The second phase of the Little Ice Age began around 1600 and lasted until around 1800. The drop in carbon dioxide, as recorded by Antarctic ice cores, that includes the result of American reforestation began in 1525 and lasted until the 1600s. The first smallpox pandemic in what is now Mexico began in 1519. I can’t confirm that was the first of the American contact pandemics, but Europeans handn’t set foot on the mainland much before that, so it must be close to the beginning.


1519: people in the Americas start dying of exotic diseases to which they have no natural immunity.

1525: global carbon dioxide levels dropped by six to 10 parts per million and stayed that way for over 75 years.

1600: temperatures drop globally, though the drop may be most severe in the northern hemisphere and stays that way for two hundred years.

There is a lot about the Little Ice Age that is debatable–why is started, why it stopped, how severe it was, all of that. That significant reforestation could follow the beginning of the pandemic by only six years itself seems questionable. However, regardless of why the carbon dioxide drop occurred, it was followed by a drop in temperature 75 years later. Carbon dioxide levels rose again shortly thereafter. Temperatures rose again about 100 years after carbon dioxide levels–that delay on warming is consistent with the principle of atmospheric lag.

Richard Nevle and his colleagues at Stanford believe that a 75 year delay in cooling is not too much for a causal relationship to exist. So there is a significant lag on cooling also.

In our modern situation, carbon re-sequestration is unlikely to be rapid–even in the best case scenario, reforestation cannot absorb more than a fraction of what burning fossil fuel released. The rest must be accomplished by peat accumulation and slow absorption by ocean water. And whatever drop in carbon levels occurs, whenever it occurs, a human lifetime could pass before the temperature follows.

We’ve got to get started.