Droughts risk rising

20:00, Mar 07 2013

The future looks drier, and that is set to affect agricultural land use, says James Renwick.

Widespread drought conditions are currently being experienced by farmers in many parts of the country, brought about by an extended spell of dry, settled weather.

While government relief is at hand in some areas, the only real relief comes from significant and regular rainfall, which is not on the horizon at the moment.

Drought is an insidious threat since it comes about not through an attention- grabbing sequence of catastrophic events, but just from day after day of "good" weather - sunny skies and no rain. A drought can take weeks and months to develop, and months or even years to recover from.

The United States drought of 2012 was unprecedented in many states and depleted water resources through the year. Drought conditions are still rated as "exceptional" right across the midwest, from Texas to the Great Lakes, nearly a year later.

Looking to the future, the risk of drought in New Zealand is on the rise. The persistent high pressure systems typical of the subtropics are already moving our way and this trend looks set to continue. The "subtropical high pressure belt" is where the world's deserts are located, and that belt is edging our way as the tropical region expands outwards under a warming climate.


Combine that with higher temperatures, increased evaporation, lower soil moisture, and we have a recipe for at least doubling the risk of drought in many of the drier parts of the country by late this century, possibly by mid-century in places. A recent report for the Ministry of Primary Industries projected an increase in drought occurrence for almost all of the country, even under an optimistic scenario for greenhouse gas emissions.

If emissions continue to run at the current high levels for too much longer, and we may be looking at a doubling of drought risk in the north and east by 2050, and a tripling (or more) of the drought risk in some regions by the end of this century.

What is quite a rare event for a farmer in Canterbury or Waikato today might be happening every few years when the farmer's grandchildren are running the property - drought is likely to become the "new normal" in some parts of eastern and northern New Zealand. Such changes imply there will be big changes needed to agricultural land use around the country.

More efficient use will need to be made of already over-stretched water resources for irrigation. Related risks will also need to be coped with - for example, more frequent dry spells and higher temperatures mean much increased risk of forest fires, as our Australian neighbours know only too well.

Chris de Freitas, of the School of Environment, at the University of Auckland noted recently that "there is no such thing as a constant climate. It's always getting drier or wetter, warmer or cooler". The climate certainly isn't constant, but relentlessly- increasing atmospheric greenhouse gas levels can mean only one thing, a much warmer and more extreme future climate.

The basics of radiation physics tell us this, and records of the past contained in ice cores confirm the near- perfect match between temperature rise and greenhouse gas increase.

There are many risks associated with climate change, as many weather and climate extremes are set to become harder to cope with. Droughts are one of the biggest climatic threats to New Zealand's agriculture- based economy, costing the country hundreds of millions of dollars or more when a significant event occurs.

For an agricultural economy like New Zealand, coping with, and being resilient to, a droughtier future will be critical.

Dr James Renwick is Associate Professor of Physical Geography at the School of Geography, Environment and Earth Sciences, Victoria University of Wellington.

The Press