Prepare for more extreme weather

01-09-2010 | Jackie Harrigan

Weather forecast for the future: More droughts interspersed with more frequent floods.

Prepare yourself for more extreme weather was the message from NIWA agricultural climate scientist Anthony Clark to farmers at a meeting of the Taumarunui Sustainable Land Management group at Returuke last month.

Despite the controversies played out in the news media, Clark told the group that scientists understood "the big-picture earth-systems science" reasonably well and when an independent group of scientists said with 80% surety that the climate was heating up, everyone should start believing them.

Temperatures have been heating up since the 1950s, he says. Clark displayed a hockey stick graph showing an exponential increase in temperature during the past 100 years.

"Despite what the climate-change sceptics are saying, the future is worrying to climatologists."

The carbon cycle theory has matured and the coupling between carbon dioxide and temperature is going to be the dominant climatic influence in the next 20-30 years, he says.

Greenhouse gas concentrations in the atmosphere have been increasing since 1750 and the globally averaged surface air temperature has gone up by around 0.6°C since the 1950s. New Zealand's climate has warmed by around 1° since the 1900s, scientists estimate.

One degree does not seem like much, but a change in the average temperature will most likely result from a change to the extremes, Clark says, when there are likely to be fewer cold days and more hot days. So droughts will be more common.

Plus a warmer atmosphere can hold more moisture, so heavy rainfall events are likely to be more intense.

As to how much additional greenhouse gas will be pumped into the atmosphere during the next 50 years and how this will affect the climate, Clark says 24 models are used to estimate the output of greenhouse gases in the future, tied closely to the level of economic growth. While all of the models are estimates only, the most radical one indicates an increase in the global surface warming temperature of 4° by the year 2100.

The NIWA map of New Zealand predicting climate changes by 2040 in terms of pasture production showed a warming and wetting of the climate to be good news for some parts of the country and bad news for others. Relative volumes of pasture production are forecast to fall in parts of the East Coast, with an increase for many of the western areas.

Generally in regions where the temperature is going to increase without too much increase in rainfall, the trends show an increase in the amount of pasture grown, a decline in nutritional feed quality, and a shorter growing season with a more pronounced feed wedge.

Farmers in the Retaruke Valley thought climate change might not be all bad for them as wetter summers and warmer winters could help grass growth.

Managing additional pasture at slightly less nutritional value and more seasonal variability will be the challenge for many New Zealand farmers, Clark says. For NZ's clover based pastures there will be an extension of winter growth but a sharper cut-off in the summer as the temperature climbs past optimal clover growth conditions.

Clark suggests it is time for NIWA to get out on to farms and start testing the theories at the local level, to see if the assumptions hold up in order to build risk awareness and to help farmers to be better prepared.

NIWA is ready to start downscaling the global assumptions - to see what climate change will mean for farmers and others on the ground in NZ, he says.

"We need to do a better job at getting everyone risk-aware, thinking about what may happen.

"It is really easy to get caught up in the bandwagon, but we need to proceed in a mature, risk-aware way."

Can your farm and community survive back-to-back droughts like 2008-09, he asked, with a short break and a false recovery, and can you manage the cumulative impacts, not just on the farm landscape but also the effect on farm income and debt servicing.

Given the uncertainty in the next 20-30 years, just being able to answer the questions the scenario throws up in a positive way helps people feel more at ease, he says.

Similarly with large rainfall events - how did your property cope in the last large even? Was there irreparable damage? Are there things you can do in a cost-effective way to mitigate the damage, for example, drainage and vegetation options, while being realistic about the costs and benefits?

Clark says climate change means that what we have seen in the past is going to become a less reliable guide to what we may see in the future. Basically, making decisions on-farm based on 20 years of climate experience may no longer be enough because farmers probably are going to be caught out by nasty surprises.

The cumulative impacts also need to be considered. How would farmers cope with a series of droughts or floods in a short space of time?

"It is not just about the biological robustness, farmers also need other buffers to shape resilience, like cash reserves and debt-to-equity ratio," Clark says. "Everyone has a risk frontier, and needs to consider the production intensity and level of risk they can handle."

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