I learnt a new word (or two) this week: physiological forcing. Sounds ominous doesn’t it? It’s like radiative forcing but relates to plants. I’m aware that some people who read my blog won’t know what is meant by radiative forcing so let’s deal with that first. The Earth gets energy from the sun in the form of short wavelength radiation. Some of this energy is lost back in to space as infrared radiation. If the Earth gains more energy than it loses, it will heat up and heat up until it is losing energy at the same rate that it is gaining energy. This is is because hotter objects lose energy faster. Prior to the equilibrium point though, when energy is coming in faster than it is being lost, we can say that there is an energy imbalance and so there is radiative forcing of the climate system. In other words, the climate system is changing due to a radiative imbalance. If this is too confusing there’s a better explanation at MIT News: Radiative Forcing.
In the same way that this energy imbalance acts as a driver of climate, plants also nudge the climate system through a process known as physiological forcing. Plants absorb carbon dioxide from the air for photosynthesis through tiny pores in their leaves called stomata. At the same time, water vapour is lost through these pores in a process called evapotranspiration. As atmospheric carbon dioxide concentrations rise, as is happening now, plants partially close these stomata. This results in improvements in water efficiency because with the closing of stomata, less water vapour is lost through their leaves. This would seem like a good thing, right? Not exactly.
Reductions in evapotranspiration cause higher air temperatures, increased continental freshwater runonff and an increased risk of drought in areas that rely on water transpired from other places. This means we can expect an increase in surface temperature as a result of reduced evapotranspiration in addition to the increase expected due to radiative forcing. From the scientific literature, Importance of carbon dioxide physiological forcing to future climate change,
Our study points to an emerging consensus that the physiological effects of increasing atmospheric CO2 on land plants will increase global warming beyond that caused by the radiative effects of CO2.
The predicted increase in water-use efficiency is already being observed and is discussed in a new paper published in the journal of Nature this week, Increase in forest water-use efficiency as atmosphere carbon dioxide concentrations rise. They found a substantial increase in water-use efficiency in forests of the Northern Hemisphere over the past two decades. They tested a number of different hypotheses for this trend and found the “observed increase most consistent with a strong CO2 fertilization effect”.
The increase in freshwater runoff is both good and bad news. Good in the sense that our freshwater resources may not be as constrained as previously thought. But bad in the sense that there is an increased risk of rain and river flooding than previously estimated.