wheat and climate change

Graham Farquhar

Research School of Biology

Australian National University

Borlaug Summit on Wheat for Food Security Borlaug100 @ CIMMYT Ciudad Obregon, Mexico, 25-28 March 2014

How will the climate change?

• Many aspects of climate:

what should we look at first?

Roderick ML, Sun F, Lim WH, Farquhar GD (2014) Hydrology and Earth System Sciences (in press).

Greenhouse effect is a radiation imbalance. Most of the extra longwave in is balanced by greater longwave out, with small increase in LE

Water is the greatest single limitation to world grain production

What will happen to precipitation? What will happen to evaporation?

• P=precipitation,

• E=evaporation (productively through the plant as transpiration, or unproductively as soil evaporation),

• P-E = runoff

• We examine a multi-model ensemble mean derived from CMIP3 models (Coupled Model Intercomparison Project Phase 3)

• using 39 runs from 20 different climate models

Roderick ML, Sun F, Lim WH, Farquhar GD (2014) Hydrology and Earth System Sciences (in press).

What will happen to precipitation and evaporation locally?

• We need granularity

• Previous research has demonstrated a “wet get wetter and dry get drier” relation in modelled output (Held & Soden, 2006)

• This has caught the scientific public’s attention

• More precisely, it was shown that (P-E) P-E, follows Clausius-Clapeyron scaling (7% increase in vapour pressure/oC), when the projected changes are averaged over latitudinal zones.

• Much of the subsequent research on impacts has been based on an implicit assumption that this CC relation also holds at local (grid box) scales

• Does it hold locally?

Roderick et al. (2014)

Conclusion re “local”

• The “Wet get wetter and the dry get drier” notion, as assessed by P-E, is not supported at the local (grid) scale by the ensemble of models

• Nevertheless the projections are for some areas to become drier, and these include some areas important to grain production, including Mexico and Central America, Chile, southern Africa, the Mediterranean & possibly southern Australia

Projected changes in precipitation

How sure can we be about these projections?

Chaotic nature of climate

• Rotstayn et al. (JGR, 2007) for the modeled annual rainfall trends in Australia over the period 1951-1996

• eight runs differed with slightly different starting conditions (in 1870)

What about observations?

Sun Roderick & Farquhar 2012 GRL Trend of the mean over 1940-2009

Virtually no change in observed global precipitation over land

What is happening to the variability of rainfall?

Sun Roderick & Farquhar 2012 GRL

Reduction in land precipitation variance over 1940-2009

On average dry have become wetter and wet have become drier

What has happened so far to evaporative demand?

• The best measure we have is pan evaporation rate.

BoM

Canberra

Airport

Will droughts get more severe with warming? Is the evaporative demand

increasing? • Around the world, pan evaporation rate has generally

been decreasing.

• Global warming is not like a hot day

• the oceans and lakes warm also, raising the humidity, so that the vapour pressure deficit is little affected on average

• The reasons for reduced evaporative demand include aerosol loading and reduced windspeed (Roderick et al., GRL 2007)

Trends - wind speed observations (Roderick, Rotstayn, Farquhar & Hobbins, GRL

2007)

Effects of [CO2] and T on growth & yield

• To what extent can we use responses to today’s variations in temperature to learn about future changes in T?

• Effects of T on assimilation rate are different at different [CO2]

• In the short term there are interfering correlations with less rainfall

• Effects on sterility or abortion-poorly understood (see Poster 132)

• Photothermal quotient

How does CO2 concentration affect the water requirements of plant growth?

• At the leaf level doubling the [CO2] is

effectively like almost doubling the rainfall. cf Wong, Cowan & Farquhar Nature 1979

• So when the [CO2] was only half its present value, say 20,000 years ago, the rainfall would only have been about half as effective as that rainfall would be today.

Impact of CO2 fertilization on maximum foliage

cover across the globe’s warm, arid environments

Randall J. Donohue, Michael L. Roderick, Tim R.

McVicar, and Graham D. Farquhar

GRL 2013

From 1982 to 2010 [CO2] increased 14% and the

responsiveness of foliage cover to precipitation increased

11% in the driest regions

Four Summary Messages

1. Many of the projected difficulties associated with adjustment or adaptation to climate change are ones that farmers have faced before. But the projections add a layer of uncertainty.

2. Increases in temperature place flowering at risk in many crops. This may require some crop production to move in a polewards direction, or to remain in place but with earlier sowing, or…. Research needed on mechanisms of heat stress.

Four Summary Messages 3. Research on water-use efficiency (which should

increase in rain-fed environments under increasing [CO2]) and drought tolerance, for present-day problems, will be vital for future climates, particularly in the context of increased demand for water, just as will research on improvement of yield potential.

4. There could be surprises and an understanding of [CO2] effects on e.g. photosynthesis, flowering and plant water relations in an ecological context would be insurance.

Thank you CIMMYT

Conclusions

• We biologists need to take into account the statistical nature of our environment

• Given the chaotic nature of climate and of good climate models, we need to examine several runs of each model as well as runs from different models

• We should be sceptical about predictions of climate driven global changes in soil water content, particularly those deriving from Thornthwaite analyses that use T rather than full energy/mass balance

Climate model P Climate model P-E