Implications for Agriculture and Forestry


The implications of both past and present climate variability and change on agriculture and forestry are the subject of impact studies, some of which have been summarized in the IPCC Third Assessment Report (IPCC, 2001b). Broad scale future impacts have already been identified. From the most well understood trends in climate during the 21st century the key trends that have been identified are

1. The continued rapid temperature increase in high latitudes of the Northern Hemisphere;

2. Further drying in Mediterranean areas, and some tropical and sub-tropical latitudes;

3. The accentuation of climate extremes as a consequence of increasing climate variability especially in sub-tropical and tropical latitudes.

All these aspects will be considered in later papers in this volume. The purpose of this contribution is to identify the underlying trends in climate change and variability during the 21st century. This will be a blend of the centennial scale trends induced by anthropogenic climate warming, on which will be superimposed decadal scale variability from the IPO and interannual variability imposed by ENSO and the NAO, of which the frequencies may be influenced by global warming.


Throughout historical time, and agrarian settlement, climate has varied. Both natural climate change and variability has occurred, which past and current agricultural and forestry systems have adapted to. Where systems in the recent geological past, such as those in the Indus valley, in this case to changing soil conditions, have not adapted, they have not survived. The record of observed climate, by instruments and proxy indicators, suggest that the rate and magnitude of change and variability has been quite modest, with centennial temperature changes globally in the order of 0.5 °C, and locally 1 °C. Natural variability because of ENSO has been a factor throughout recorded history.

However, the magnitude and rates of change that are projected for the 21st century fall outside that range. The 90% confidence range of global warming is in the range of 2 to 4.5 °C (Wigley and Raper, 2001). Although agriculture and forestry might adapt given a modest rate of climate change, the rapidity of projected change is unprecedented in the last 10,000 yr. The current rate of global warming since the mid-1970s has been at 0.2 °C per decade, which is consistent with the lower projected warming rates for the 21st century.

The United Nations Framework Convention on Climate Change (United Nations, 1992) main purpose is to reduce the growth of greenhouse gases in the atmosphere and stabilize climate.

"the ultimate aim of this Convention... is to achieve... Stabilisation of greenhouse gas concentrations in the prevent dangerous climate change1"

The natural greenhouse effect keeps the planet and biosphere at an equable temperature for planetary processes to operate (e.g. Salinger et al., 2000).

Climate, agriculture and forestry are thus inextricably linked. The mean surface air temperature of the earth can be used as a measure of the stability of the climate system. It responds to energy inputs, and cycling processes such as the hydrological (water) cycle. Temperature is part of the process of life systems. Over the last 420,000 yr climate has varied by 6 °C between glacial and interglacial periods, with the most rapid change being about 1 °C per century. Temperature increases or decreases outside these ranges will create an unstable climate, as parts attempt to adjust to rapidly changing temperatures. Native forests take centuries to adjust their range, and agriculture would face almost impossible adjustments.

The current rate of global warming is 2 °C per century, and this rate is projected as a lower range for the remainder of the 21st century. Thus increases in greenhouse gases released by human activities are creating a potential situation of dangerous climate change where the stability of agriculture and forestry systems is threatened. Greenhouse gases are likely to double during this century. This could bring with it unknown climate surprises and their impacts, such as flooding from unmanageable catchments and inundation of land areas due to storm surges and sea level rise.

Adaptation strategies are going to be of crucial importance. These can range from traditional to new technologies. Traditional management practices such as intercropping, mulching and agroforestry will be important. Changes in agronomic practices such as earlier planting or cultivar switching are simple adaptive strategies for the tropics. Earlier planting and sowing, shorter rotations and larger spacing in areas undergoing drying and use of shelterbelts can be used in temperate regions. Understanding of impacts, modeling and improved spatial measurement of agriculture and forestry will provide new methods of adaptation. The improvement of seasonal climate forecasting will increase preparedness and risk management on seasonal to interannual time-scales for increasing climate variability. All these strategies and others discussed in later papers in this volume will be of critical importance to cope with the increasing climate variability and change of the 21st century to prevent these variations being dangerous to agriculture and forestry.

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