Climate reality - actual and expected

Abstract

New Zealand average surface temperatures have increased by 0.7 °C since 1871. In the last quarter of the 20th century, more prevalent west to southwest flows occurred, accompanying a higher incidence of El Niño events. This resulted in annual rainfall decreasing in eastern areas of the North Island. As well as the global warming signal, interannual to decadal climate variability is a strong feature of east coast dryland climates. The El Niño-Southern Oscillation, (ENSO), through El Niño/ and La Niña episodes, drives climate variability seasonally. The recently described Interdecadal Pacific Oscillation (IPO) shifts climate every one to three decades and changes precipitation averages in these areas. These features of the climate system leave east coast dryland farming open to considerable climate variability. Records of potential soil moisture deficit (PSMD) around Napier and Ashburton show that significant PSMD developed in these regions by 1 December, in 50 to 85% of years with severe deficits in 20 to 55% of years. These deficits build as summer progresses. El Niño events intensify, whilst La Niña episodes normally ameliorate these conditions on seasonal time scales. The IPO climate shifts significantly change the dryness of the soil of these areas, with the transition from negative to positive phases increasing PSMD by 35 to 50 mm. Climate change over the next few decades will be driven by the underlying trend of global warming. For New Zealand, this will be a warming of about 0.2 °C per decade. The latest scenarios and climate model results indicate that westerly circulation is likely to strengthen over New Zealand, with a drying of east coast climate in the order of 10% by 2080. These will cause an increase in PSMD in the order of 20 to 30%. ENSO and IPO variability will be a continuing feature of New Zealand climate in coming decades. East coast dryland farms experience substantial climate variability. As climate warming continues in the decades of the 21st century, these areas will become increasingly stressed as potential evapotranspiration (PET) rates increase, particularly when the IPO next changes phase and during El Niño events. Climate forecasting is an exciting new technology that will give farmers early warning and increase preparedness for dry seasons ahead, allowing them to make key strategic decisions. A mixture of new and traditional technologies will also assist, such as intercropping and use of seasonal climate forecasting. Despite this, dryland farming systems are likely to become increasingly limited owing to low rainfall and high potential evapotranspiration rates.