Plants and Climate Change: It’s Not That Simple

by Philip Duffy

There has been a lot of discussion recently about plants and climate change, and rightly so. Deforestation is a major cause of climate change, and at the same time climate change can alter plants in ways that seem to be making the problem worse.

A new paper by Long Cao of the Carnegie Institution at Stanford University adds to the discussion by highlighting a subtle but important response of plants to increasing carbon dioxide (CO2) in the atmosphere: reduced evaporation. This turns out to have complex and potentially important effects on climate.

The most widely-discussed issue involving plants and climate change is deforestation. Cutting down trees, particularly in tropical regions, is the second largest source of greenhouse gas emissions (after fossil-fuel burning), contributing 12 percent of total emissions, according to a 2009 study published in the journal Nature Geoscience. Conversely, planting trees is a commonly-used method for offsetting greenhouse gas emissions — although it is not as effective as many people suppose, as demonstrated by a 2007 study in the Proceedings of the National Academy of Sciences. For one thing, the carbon-absorbing effect of trees is temporary; in addition, trees darken the surface and warming from this can be greater than the cooling effect from absorbing carbon.

Despite significant carbon emissions from deforestation, many people don’t realize that on the whole, terrestrial ecosystems (plants and soils) remove lots of carbon from the atmosphere—about 29 percent of all human emissions, by the latest estimate.

The sum-total, global effect of plants and soils is to remove carbon from the atmosphere. Regional deforestation, however, emits greenhouse gases, and climate change would proceed more slowly if tropical deforestation were stopped. This would also benefit biodiversity, since perhaps 50 percent of the world’s species live in tropical forests.

Spatial observations are critical to determining patterns and variability in the carbon cycle. This image shows a global map of terrestrial net primary production. (source: Global Carbon Project)

 

It’s helpful that plants and soils offset some of our carbon emissions and help slow climate change. However, there’s reason to think that climate change itself is weakening this effect. That is, the warmer it gets, the less CO2 plants and soils will absorb. This of course adds to the warming, creating a positive feedback loop. Computer modeling studies suggest that this effect could be important.

Besides responding to warming and other aspects of climate change (like changes in precipitation), plants also respond directly to increased CO2 in the atmosphere. The best-known of these effects is “CO2-fertilization”, in which increased atmospheric CO2 stimulates plant growth. In principal, this could be an important negative feedback on climate change — one that slows warming. However, experiments suggest that in the real world this effect not as strong as it could be, because other factors such as a lack of water or nutrients often limit plant growth. In fact, models indicate that CO2-fertilization will be overwhelmed by other effects that act in the opposite direction, such as reductions in biomass caused by drying.

As Cao’s paper discusses, one of the important ways in which plants affect climate is by pulling moisture out of the soil and increasing evaporation. This does not happen as much in a higher-CO2 environment, because with more CO2 present, plants don’t need to try as hard to get CO2 from the atmosphere. This gives water vapor less chance to escape and thereby reduces evaporation.

This in turn tends to reduce relative humidity and decrease low-cloud cover over land. Because low clouds reflect sunlight back into space, reducing low-cloud cover causes warming. So, through a series of interactions, the CO2-induced reduction in evaporation through plants amplifies warming. Cao and his coauthors suggest that, absent other changes, this effect could be quite significant: amplifying warming between 10 and 15 percent globally, and much more in some regions. (Less in other regions, of course, but those are mainly over oceans.)

What’s probably just as important to humanity is an increase in surface runoff: excess moisture that flows into streams and rivers towards the ocean. If evaporation through plants decreases, this would actually boost runoff, because over land there would be more precipitation and less evaporation (hence an increase in moisture available to run off). In the real world, this would add to the increase in runoff expected from warming alone.

Early snowmelts have led to changes in the hydrological cycle in Montana

 

Stepping back from these interesting (and subtle) specifics, what is the overall significance of these plant responses to CO2? First, Cao’s study is a great example of why the best use of models in science is to educate our intuition. The paper describes an effect that seems counterintuitive at first (less evaporation results in more runoff???), but upon further reflection makes sense. Are the numbers in this paper right? Almost certainly not. Is the basic idea correct? Probably. More importantly, this paper reminds us of the many complex interactions among biological organisms and the atmosphere that are triggered by releasing large amounts of carbon into the atmosphere.

We should keep this in mind when considering other insults to the system, particularly “geoengineering.” It also reinforces the idea that the impacts of climate change that may be the most difficult to cope with may be changes in the hydrological cycle rather than changes in temperature. These hydrological impacts are being felt even now in semi-arid regions such as the US West, and are expected to become more severe in the future.