7 November 1998

In the Hotseat…

Global warming may favour European farmers but it wont all be plain sailing, according to Professor Martin Parry, of University College London, the Bawden lecturer at this months Brighton Conference on Pests & Diseases.

Do scientists now agree that global warming is a reality?

Yes, it is now accepted that natural causes alone are not sufficient to explain the rate and pattern of long-term climate change experienced during the past century. There is also broad agreement that increases in greenhouse gases will continue well into the 21st century.

So what has been the effect on our climate?

The records show a warming by up to 0.6íC since the late 19th century. This has not been uniform but is strongest over the continents between 40 degrees N and 70N. There has been a long-term increase in precipitation at high latitudes in the northern hemisphere and a sharp decrease in precipitation since the 1960s over the sub-tropics and tropics.

What about the future?

These trends will continue so, for example, for Europe, the projections indicate an increase in mean annual temperatures by from 1íC to 2íC by the year 2100.

How will these changes affect crop output?

In general, increases in atmospheric carbon dioxide (CO2 ) should be beneficial for plants because the more CO2 available for the plant to convert into carbohydrate, the better plant growth. In controlled experiments, yield increases of up to 36% have been obtained for wheat, rice and barley when ambient CO2 is doubled. At the same time, little is known about the effect on quality but it is likely protein levels will be reduced.

So more CO2 sounds like good news, but what about those higher temperatures?

Generally they will lead to lower yields in plants like cereals which have a discrete life cycle but higher yields in root crops and grass which continue to grow throughout the season.

Increased temperatures may also lead to reduced moisture availability, limiting yield. However, where production is currently limited by the length of the growing season, a small increase in the mean annual temperate is likely to be good news.

On balance winter cereals yields are predicted to increase across most of Europe at a rate of 0.13-0.2t/ha a decade. Potato yields may increase by as much as 35% because of the lengthening growing season.

Northern Europe will also benefit from a reduced risk of early and late frosts and improved maturation rates.

And what about weeds, pests and diseases?

Weeds are expected to benefit from higher CO2 concentrations while rainfall increases in the northern half of Europe will probably be linked to higher air humidity and prolonged leaf wetness – factors favourable to early disease. A northwards shift in the distribution of certain pests is likely.

So northern Europe may be a winner from the greenhouse effect?

No, it would be a mistake to believe Europe will be a winner in a game of winners and losers because climate change is a global issue requiring a global co-operative response.

Offsetting any increases in yield in Europe will be progressive decreases in other parts of the world. Predictions are for an extra 36 million people to be put at risk of hunger by 2020 at a time when, without global climate change, these numbers should actually be falling.