A little good news on climate change

Climate change is with us, but the future impact on the wine sector may not be quite as dramatic as many fear.

A four-year study in Shiraz vineyards around Mildura suggests that, for that region at least, increased temperatures and levels of CO₂ do not have a significant impact on grape yield or quality.

‘Certainly nothing screamed out at us’, said project leader Dr Michael Treeby. ‘The grapes we produced were more-or-less the same as the grapes we produce now, and that’s not going to change very much.

‘And when we followed that through all the way to the wine, most of the time nobody could tell whether the wines came from vines that were heated or vines with higher CO₂ or vines with both.’

The most important part of that sentence is the last bit. While there has been a bit of research examining the likely impact of higher temperatures, until now very little has looked at what elevated CO₂ levels might mean.

And yet it’s the obvious question, because while CO₂ is causing the temperature increases, it also has an impact on other things, notably photosynthesis. With higher CO₂, plants fix more carbon.

Photo: Ewen Bell / Wine Australia

‘We didn’t know the implications of that’, said Dr Treeby, who is Group Leader for Horticulture Production Sciences with Agriculture Victoria Research.

‘Do you get the same effect when you have both higher temperatures and CO₂, which is what viticulture is going to have to contend with in 2050? Does extra CO₂ mitigate temperature increases in some way?’

We now have some of the answers.

‘We know that most of the phenological effects – from budburst right through to harvest – are temperature driven, but when we start looking at leaf function, the amount of carbon being fixed, that’s all driven by CO₂. Temperature is far less influential. So, the carbon balance of those vines is changing.

‘We also know that the amount of water used by the vines is driven by temperature – there are no great shocks there – but if vines are fixing more carbon they are inherently more efficient at it. That’s surprising, but it also offers a little bit of hope and some opportunities to use water more efficiently.’

The next question is whether similar results would be found in other regions, particularly those with a cooler climate. Dr Treeby can’t currently answer that, but he notes that the results were consistent across the four years of the study, which included variable conditions.

Interestingly, elevated CO₂ was associated with an extra two kilograms of fruit per vine per season overall, but that effect was dwarfed by a factor of five by the seasonal effect, prompting Treeby to remind us that ‘the biggest thing that influences anything is the weather’.

‘Weather means all those unique combinations of temperature, humidity, cloud cover and so on that are beyond our control. We applied a uniform 250 parts per million higher CO₂ and a +2° temperature relative to ambient temperature across the board. That’s a climate change, not weather change.’

The project, which involved input from the CSIRO, was both comprehensive and complicated, if only for the logistical difficulties out in the field of getting controlled amounts of CO₂ directly to the vines and monitoring air temperature constantly.

Funding was received from Wine Australia, the Victorian Government and CSIRO. The project’s final report can be downloaded here.