If we would pump aerosols in the stratosphere to artificially cool the Earth and thereby compensate (part of) the current climate warming, we would be permanently living under a slight sunshade.

That would mean in a futuristic world it may take you a couple of minutes longer to get that nice spring tan. But it may also hurt agricultural productivity, some have suggested. Plants after all need sunlight as the energy source for photosynthesis, or growth.

Well, a group of four atmospheric scientists of Carnegie Institution for Science and Harvard University have investigated the matter and think a slight decrease in insolation would not be the crops primary concern.

Earth is green because it is a blue planet

If life was all about getting enough sun exposure then why isn’t there any at Venus. Indeed, the presence of water – the one true key to (plant) life.

So indeed there is also good reason to assume precipitation changes are what could really affect agriculture under unabated climate change – and any factor stabilising both temperature and rainfall would likely do good work maintaining the ecological status quo that also the global food supply would benefit from most.

The Americans – who published their findings on Sunday in Nature Climate Change – ran two different climate models, CAM3.5 and HadCM3L – the one devised by the US National Center for Atmospheric Research and the other by the UK Met Office’s Hadley Centre and simulated a doubling of atmospheric CO2 concentrations, temperature-compensating stratospheric solar radiation management (SRM) geoengineering – and compared precipitation changes. Both models had largely similar results.

As shown above in the HadCM3L run (in which high CO2-fertilisation effect is supposed) locally (the Arctic, Russia, Brazil, continental Africa, Antarctic Peninsula) the doubling of atmospheric CO2 [see ‘climate sensitivity’ for further reading] would lead to >5 degrees Celsius warming. As a response [and as consequence of an increase in the general circulation] on average dry regions would get drier, and wet regions would become wetter. SRM geoengineering would be able to even out most of this climate change, both for temperature and precipitation, the model suggests. [Please note the model shows a lot of Amazon heating, but little (annual mean) Amazon drying – which does not correspond well with findings from the Pleistocene climate record or present-day (2005, 2010 droughts) observations. Let’s hope the model is right.]

These climate model outcomes the researchers then translated to crop growth conditions [even including CO2-fertilisation effects] for maize, wheat and rice, for the different geographical regions. Although the researchers conclude on average doubling CO2 without SRM geoengineering leads to a larger decline in food productivity that doubling CO2 with geoengineering – this is not a direct conclusion from the above image, which it easier to interpret, but falsely suggests one could grow rice in the Siberian tundra – and that doubling CO2 would be a good idea. On the Indian subcontinent and in the African tropics normal climate change would lower the rice yield, while adding SRM geoengineering would be beneficial.

It is therefore important to also take a good look at the below picture, which better represents total agricultural productivity for each of the three prime food crops – and which shows the main production latitudes for maize, wheat and rice (at moderate CO2 fertilisation):

Unabated climate change (top) seen on average negatively affecting crop yield, even including some CO2 fertilisation. Including (bottom) SRM geoengineering on average leads to increased agricultural productivity, the model suggests.

And if you like it simpler, take a look it this one – which (supposing moderate CO2 fertilisation) – simply states adding SRM geoengineering affects crop yield by between -10 and +30 percent, with only local declines for rice and only benefits for maize and wheat productivity:

The study concludes SRM geoengineering is unlikely to negatively impact agricultural food productivity, especially since it compensates part of the damaging effects of unabated climate change to this food production.

This study of course does not take away very different concerns related to stratospheric aerosol SRM geoengineering, like possible damage to the ozone layer [which in turn would be good news if you hate waiting for that spring tan] and the fact that allowing CO2 concentrations to keep rising presents other problems, like the necessity to never stop with the active process of SRM geoengineering, and increasing ecological damage caused by ocean acidification.

But aren’t we forgetting about something else here? Crops and geoengineering – doesn’t that sound familiar? As the slightest of albedo changes can have quite a temperature impact, how do all these agricultural changes – either under unabated climate change or in a stratospheric SRM world – in turn affect the atmosphere?

© Rolf Schuttenhelm | www.bitsofscience.org