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Importance of carbon dioxide physiological forcing to future climate change

Long Cao, Govindasamy Bala, Ken Caldeira, Ramakrishna Nemani, and George Ban-Weiss · Proceedings of the National Academy of Sciences 107, 9513-9518 · 2010

Key finding. In response to doubled carbon dioxide, the radiative effect raises mean surface air temperature over land by 2.86 ± 0.02 K and the physiological effect on land plants adds a further 0.42 ± 0.02 K — and for runoff the physiological effect is the larger of the two, contributing 8.4 ± 0.6% against the radiative effect's 5.2 ± 0.6%.

A three-by-three grid of world maps. Rows are the radiative effect of carbon dioxide, its physiological effect on plants, and the two combined; columns are surface air temperature in kelvin, runoff in millimetres per day, and near-surface relative humidity. The radiative row is uniformly warm across the continents; the physiological row shows weaker warming but comparably strong runoff increases, concentrated over the Amazon, central Africa and Southeast Asia.
Compare the middle row with the top. Plant stomatal closure produces far less warming than the radiative effect but a comparable — in fact larger — increase in runoff, and it does so over exactly the tropical forests where transpiration matters most. Figure 2 from Cao et al. (2010), Proceedings of the National Academy of Sciences 107, 9513-9518. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Carbon dioxide changes climate two ways. It traps longwave radiation, which is the effect everyone models. It also makes plants partly close their stomata, reducing transpiration, which warms and dries the land surface independently of any radiative change.

The second effect is routinely omitted or treated as secondary. This paper asked how large it actually is, by separating the two in a coupled land-atmosphere model.

What did we find?

Separating the two effects in the NCAR coupled Community Land and Community Atmosphere Model gives land warming of 2.86 K radiative, 0.42 K physiological, and 3.33 ± 0.03 K combined.

For runoff the ordering reverses. The radiative effect adds 5.2% mainly by increasing precipitation over the continents; the physiological effect adds 8.4% mainly by cutting evapotranspiration from them.

Combined, runoff rises 14.9 ± 0.7% — so roughly half the projected increase comes from a mechanism that has nothing to do with trapping heat.

The two effects reach the same outcome by opposite routes: one adds water from above, the other stops plants returning it to the air.

Why does it matter?

It makes plant physiology a first-order term in projections of the land water cycle rather than a refinement. A model that omits stomatal closure understates land warming by about 15% and understates runoff increase by more than half.

That has direct consequences for flood and water resource projections, since runoff is what rivers carry — and the omitted term is the larger contributor.

It also means carbon dioxide cannot be fully represented by its radiative forcing, which is the same reason solar geoengineering cannot exactly reverse it: sunlight does not close stomata.

Citation

Long Cao, Govindasamy Bala, Ken Caldeira, Ramakrishna Nemani, and George Ban-Weiss (2010). Importance of carbon dioxide physiological forcing to future climate change. Proceedings of the National Academy of Sciences 107, 9513-9518.

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