GREENBELT, Md. -- A new NASA computer modeling effort has found that additional growth of plants and trees in a world with doubled atmospheric carbon dioxide levels would create a new negative feedback – a cooling effect – in the Earth's climate system that could work to reduce future global warming.
The cooling effect would be -0.3 degrees Celsius (C) (-0.5 Fahrenheit (F)) globally and -0.6 degrees C (-1.1 F) over land, compared to simulations where the feedback was not included, said Lahouari Bounoua, of Goddard Space Flight Center, Greenbelt, Md. Bounoua is lead author on a paper detailing the results that will be published Dec. 7 in the journal Geophysical Research Letters.
Without the negative feedback included, the model found a warming of 1.94 degrees C globally when carbon dioxide was doubled.
Note that in models of this sort there are many unknowns and probabilities. The warming from CO2 could be smaller or larger than this study used. The feedbacks from plants could be different for a number of reasons.
Plant growth causes both positive and negative feedbacks on temperature. This study finds the negative feedbacks will dominate.
An example of a positive feedback would be if warming temperatures caused forests to grow in the place of Arctic tundra. The darker surface of a forest canopy would absorb more solar radiation than the snowy tundra, which reflects more solar radiation. The greater absorption would amplify warming. The vegetative feedback modeled in this research, in which increased plant growth would exert a cooling effect, is an example of a negative feedback. The feedback quantified in this study is a result of an interaction between all these aspects: carbon dioxide enrichment, a warming and moistening climate, plants' more efficient use of water, down-regulation and the ability for leaf growth.
When CO2 is high plants do not have to open up their pores for as long to let in CO2. So they lose less water. Hence the comment about "plants' more efficient use of water". That means plants will evaporate less water into the atmosphere. It also means that plants will become less water-limited in their growth. Curiously, the lower water need should cause forests to expand into deserts. Though higher temperatures could cause droughts in some areas that cut plant levels. Hard to say how this all shakes out globally.
While I expect Peak Oil to happen fairly soon I am far less clear on Peak Natural Gas or Peak Coal. Some argue that we have far less coal left than commonly believed. But we do not know how coal production would respond to sustained higher prices. Maybe 90% of the extractable coal will be used by 2070 (PDF). Then again, maybe the amount of coal available would double in response to less than a doubling of coal prices.
Contrary to conventional belief, as the climate warms and growing seasons lengthen subalpine forests are likely to soak up less carbon dioxide, according to a new University of Colorado at Boulder study.
As a result, more of the greenhouse gas will be left to concentrate in the atmosphere.
"Our findings contradict studies of other ecosystems that conclude longer growing seasons actually increase plant carbon uptake," said Jia Hu, who conducted the research as a graduate student in CU-Boulder's ecology and evolutionary biology department in conjunction with the university's Cooperative Institute for Research in Environmental Sciences, or CIRES.
The study will be published in the February edition of the journal Global Change Biology.
Working with ecology and evolutionary biology professor and CIRES Fellow Russell Monson, Hu found that while smaller spring snowpack tended to advance the onset of spring and extend the growing season, it also reduced the amount of water available to forests later in the summer and fall. The water-stressed trees were then less effective in converting CO2 into biomass. Summer rains were unable to make up the difference, Hu said.
While not mentioned in this press release, outright drought is especially problematic for use of trees to capture CO2. Pine needs water in order to produce resin that protects against beetle infestation (yes, trees have active defensive mechanisms against pests). Without enough water pine trees will get killed by beetles. Obviously dead trees release CO2 rather than absorb it. Warmer winters make this problem worse by reducing snow pack and also by not killing the beetles. More beetles survive warmer winters and cause more damage to trees.
While longer seasons and higher CO2 will increase plant growth in many areas that won't happen where drought and reduced summer water run-off makes water a bigger limiting factor than CO2 for plant growth.
Global temperatures over the last 10 years haven't risen as fast climate models predicted based on rising CO2. A new report might explain this result: a decline in water vapor appears to have slowed the warming.
A decrease in water vapor concentrations in parts of the middle atmosphere has contributed to a slowing of Earth’s warming, researchers are reporting. The finding, they said, offers part of the explanation for a string of years with relatively stable global surface temperatures.
Anyone know what mechanism might be responsible for the changes in water vapor reported in this study? Any reason to expect a continuation or reversal in the water vapor decline?
Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here, we show that this acted to slow the rate of increase in global surface temperature over 2000 to 2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% compared to estimates neglecting this change.
Will the decline in water vapor continue or reverse?
Note how a recent NASA announcement about record temperatures shows several of the latest 12 years all having about the same temperature. That's not an upward trend. The report above suggests why.
WASHINGTON -- A new analysis of global surface temperatures by NASA scientists finds the past year was tied for the second warmest since 1880. In the Southern Hemisphere, 2009 was the warmest year on record.
Although 2008 was the coolest year of the decade because of a strong La Nina that cooled the tropical Pacific Ocean, 2009 saw a return to a near-record global temperatures as the La Nina diminished, according to the new analysis by NASA's Goddard Institute for Space Studies (GISS) in New York. The past year was a small fraction of a degree cooler than 2005, the warmest on record, putting 2009 in a virtual tie with a cluster of other years --1998, 2002, 2003, 2006, and 2007 -- for the second warmest on record.
If the water vapor stays at current levels then rising CO2 likely will cause a resumption in average air temperatures.
I am in the process of reading more about climate science. Anyone have suggested reading? I'm not looking for political diatribes or books about the politics of climate change. I'd like to develop a better understanding of climate science.
Update: The key question in my mind about this report: Did warming cause the decrease in water vapor in the stratosphere? Or is it coincidental. To put it more succinctly: Was the decrease in stratospheric water vapor a negative feedback of global warming? If you want to predict the future of the climate you have know all the major feedbacks and predict their future behavior. NOTE: I fixed this. changed "increase" to "decrease" in this paragraph.