Guest Post By Ken Caldeira Of Stanford University

In response to the announcement

Water evaporated from trees cools global climate

and a press release titled

 “World-lead​ing scientists will shed new light on climate-ch​ange mitigation in Garmisch-P​artenkirch​en, Germany, 18-23 September”

sent to us by Andy Revkin, Ken Caldeira of Stanford University sent the following e-mails, which he has permitted me to post on my weblog. 

A  different press release (sent out by Tanji Suni of Finland; Unfortunately, I do not have the url for it) reads in part [highlight added]

World-leading climate scientists gather in Germany next week to present new research on how Earth’s land areas – forests, soils, permanently frozen regions, deserts, croplands and pastures – affect the global climate. The major international conference, organised by the Integrated Land Ecosystems-Atmosphere Process Study, will bring together 400 scientists to shed new light on climate-change mitigation.

Conference organiser, Academy Professor Markku Kulmala from University of Helsinki, Finland said: “Humans have commandeered over 75% of the Earth’s land outside of Greenland and Antarctica. How we manage this land this century will be a major factor in whether we can stabilise global greenhouse gas emissions.” The conference, to take place in the Bavarian town of Garmisch-Partenkirchen, will outline the latest research to combat climate change.

Although there has been much discussion about greenhouse gas emissions from burning fossil fuels, the next largest source of emissions is how humans have changed the global landscape. Agriculture and deforestation began thousands of years ago. Dr Julia Pongratz from Stanford University, USA,  explains: “Deforestation has resulted in emissions of CO2 into the atmosphere, but clearing the forests may also have cooled the climate because forest is darker and heats up more than agricultural and pasture land.” The increase in atmospheric CO2 caused by human land-cover change became significant long before the Industrial Revolution and by present day its overall effect amounts to about 20 ppm: human changes to land cover appears to explain about one eighth of the observed 1 K global warming.

“The decision of farmers concerning where to settle and farm in the historical past has enhanced the mitigation potential of future reforestation of this land: because farmers generally chose to deforest land that was more productive than average, reforesting this land can absorb large amounts of CO2 and have a strong cooling influence”, Dr Pongratz concludes.

Ken’s e-mail reads

Date: Tue, 20 Sep 2011 07:17:50 -0700

From: Ken Caldeira To: Andrew Revkin Cc: Roger A Pielke Sr    George Ban-Weiss    Julia Pongratz 

Subject: Re: World-leading scientists will shed new light on climate-change
   mitigation in Garmisch-Partenkirchen, Germany, 18-23 September

Yes, Julia below is my post-doc.

The paper she is referring to is attached [I have added provided the urls for the papers mentioned at the bottom of this post]. I also attach another paper I was involved in that came out in 2007.

In the 2007 paper we looked a hypothetical deforestation of everything north of 50 degrees latitude and found that it produced a net warming influence, because the albedo influence won out over effects of carbon storage and evapotranspiration.

However, a more detailed look came to the opposite conclusion, because farmers did not just use average land, but they chose land that was both more productive and less snowy than average.  More productive means higher carbon storage, so more carbon is lost when this land is converted from forest to agriculture.  Less snowy means that there is less of a change in surface reflectivity (albedo) when the conversion is made. Both of these things resulted in historical deforestation in boreal regions having a warming influence, not a cooling influence as suggested by our earlier study.

There was nothing wrong with the earlier study. It just shows that you cannot apply simple thought experiments too directly to real world situations.

Julia made a YouTube video talking about this paper:

I also include another paper I worked on with Julia on effects of historical wars and famines on atmospheric CO2 (minor, but that’s not what others e.g. Ruddiman have been saying).  Julia also did a video on that one:

We shouldn’t forget in all of this that climate change is only one of several important factors affecting our environment. The biggest changes to date have really come from direct effects of conversion of land from natural ecosystems to agricultural systems, with the resulting loss of biodiversity. The oceans have also undergone vast ecological change as a result of overfishing. So far, the environmental damage from climate change is a pittance when compared with the direct damage to these systems.

Even into the future, cutting down a forest is likely to do more environmental damage directly (i.e., by destroying the forest) than indirectly (i.e, through the impact of CO2 releases on climate and thereby on other more distant ecosystems).

So, it is important to think about the influence of land-cover change on climate, but climate does not have to be the lens through which everything is viewed.

If we think from the broader perspective of environmental protection, preventing climate change can help protect our environment, but maintaining and restoring natural ecosystems on land and in the oceans are also very important. (What if we prevented climate change, but then there were no natural ecosystems left to be affected by climate change?)

Thus, undisturbed boreal forests should be protected even where cutting down those forests would have a cooling influence.

A follow-up e-mail reads

Date: Tue, 20 Sep 2011 07:23:01 -0700

From: Ken Caldeira To: Andrew Revkin Cc: Roger A Pielke Sr    George Ban-Weiss    Julia Pongratz

Subject: (one more thing)

Most previous studies had been of the form of Bala et al or Pongratz et al of the previous email, where land-type A is changed to land-type B and then you see how a climate model reacts.

The study led by George is part of a research program aimed at picking apart the different components of the land-cover change to understand the mechanisms.

When land-type A changes to land-type B, typically, the amount of evaporation changes, the reflectivity changes, the surface roughness changes, and so on. Which of these effects dominate the climate response?

So, we are trying to look at these one-by-one to understand the influence of each effect.

And, as with the study of boreal deforestation, the basic plan is to start with very idealized studies and work towards more “realistic” studies. Thus, the paper led by George is the first paper from a new research direction aimed at teasing apart the importance of different causal chains in the relationship between land-cover change and climate change.

Ken Caldeira

Carnegie Institution Dept of Global Ecology
260 Panama Street, Stanford, CA 94305 USA
+1 650 704 7212

The papers Ken refers to are

J. Pongratz, C. H. Reick, T. Raddatz, K. Caldeira, and M. Claussen (2011), Past land use decisions have increased mitigation potential of reforestation, Geophys. Res. Lett., 38, L15701, doi:10.1029/2011GL047848.

Julia Pongratz, Ken Caldeira, Christian H. Reick andMartin Claussen, 2011:Coupled climate–carbon simulations
indicate minor global effects of wars and epidemics on atmospheric  CO2 between an d 800 and 1850
. The Holocene 1–9

G. Bala, K. Caldeira, M. Wickett, T. J. Phillips, D. B. Lobell, C. Delire, and A. Mirin, 2007: Combined climate and carbon-cycle effects
of large-scale deforestation. PNAS. April 17, 2007

source of image

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