April 28, 2010

In Defense of Humans

Imagine if the global annual average temperature were about 5°F colder than it is presently.

Not quite sure how to? OK, consider this: During the Little Ice Age—a period extending from about the 1500s to the mid-1800s and thought to be one of the coldest periods during the past 10,000 years or so—the earth’s average temperature may have been 2-3°F colder than present. Associated with the Little Ice Age are all sorts of human calamities—widespread crop failures, plagues, famines, population declines, glacial encroachments, etc. For a collection of descriptions of all the fun times that a colder climate brings, take a gander at the Wikipedia page on the Little Ice Age. After spending a few minutes there, you’ll see that these were not high times for humans.

Now, consider a temperature decline twice that much. That can’t be good.

Yet that’s apparently where we would be had human ingenuity not come along.

According to a new study just published in the journal Geophysical Research Letters, the global climate would be about 5°F colder than present were it not for human carbon dioxide and methane emissions.


University of Wisconsin-Madison’s Gwenaëlle Philippon-Berthier, Stephen Vavrus, and John Kutzbach, along with University of Virginia’s William Ruddiman used a climate model to examine what the earth’s climate and vegetation may have been like absent any human greenhouse gas (GHG) emissions. And this is what they found:

Based on the GHG levels inferred from previous interglacials… we estimate that in the absence of all anthropogenic GHG emissions, the current climate would be approximately 3.14K [5.7°F] cooler. We find that a substantial amount of this climate change (nearly 0.5K) is attributable to vegetation feedbacks… Moreover, the plant physiology feedbacks add to the dynamic vegetation feedbacks…in leading to significant regional changes: more permanent snow cover (in the NH), less high-latitude vegetation, colder extratropics, and less tropical vegetation.

Without us and our development of agriculture and energy from fossil fuels, the world’s climate would not be able to support us in the numbers that it does today. Talk about a positive feedback! We can’t think of a better one.

And not only do we humans benefit, but so to does the earth’s vegetation. Philippon-Berthier and colleagues calculated that as a result of the colder and drier conditions, along with lower levels of atmospheric carbon dioxide (a plant fertilizer), terrestrial photosynthesis would decline by 39%, leaf area would decline by 30%. In the Northern Hemisphere mid-latitudes, forest cover would decline by 60% and grassland area would decline by 17%. In the high latitudes, the area of boreal forests would drop by 69% while the area of polar desert would increase by 286%. And in the Tropics, grass area would decline by 3%, forest area by 15%, and the area of bare ground would increase by 344%.

Figure 1 shows the modeled change in the distribution of the world’s forests and grasslands under climate conditions with no human GHG emissions.

Figure 1. Total grass (top) and tree (bottom) differences (percentages) from current climate conditions to conditions with no human GHG contributions (source: Philippon-Berthier et al., 2010).

One of the study’s co-authors, William Ruddiman has been pushing the idea for some time now that human activities have quite possibily been responsible for heading off the beginning of the next ice age. This latest study suggests another benefit of human development on the climate—warmer, greener conditions.

The bottom line here is that the world would surely would be a colder, drier, and browner place had we not come along and warmed things up a bit (and thrown in some atmospheric fertilizer along the way).

So instead of decrying our influence on the climate (as some of us are keen on doing), we all should be celebrating it!


Philippon-Berthier, G., et al., 2010. Role of plant physiology and dynamic vegetation feedbacks in the climate response to low GHG concentrations typical of the late stages of previous interglacials. Geophysical Research Letters, 37, L08705, doi:10.1029/2010GL042905.

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