Land use 'alters climate'

Reforestation in snowy areas can change the climate

By Alex Kirby BBC News Online environment correspondent

The way humans alter the surface of the Earth may be a key factor in climate change, scientists believe.

They say land-use changes are probably just as important as greenhouse gas emissions.

They think tropical land surface changes are probably a greater influence on climate than the seasonal El Nino weather disturbances in the Pacific.

And they suggest a new formula for measuring all human-caused climate influences.

The scientists, whose work was funded by the US space agency Nasa, published their findings in the Philosophical Transactions of London's Royal Society, the UK's national academy of sciences.

They say changes like urban sprawl, the destruction and planting of forests, and farming and irrigation all have a strong effect on regional surface temperatures, precipitation and larger-scale atmospheric circulation.

So human-caused (anthropogenic) land changes, especially in North America, Europe and south-east Asia, redistribute atmospheric heat regionally and globally.

Centuries of change

The researchers say these changes "may actually have a greater impact on climate than that due to anthropogenic greenhouse gases combined".

The lead author of the study is Dr Roger Pielke Sr, an atmospheric scientist at Colorado State University, US.

Irrigation alters rain patterns

He said: "Our work suggests that the impacts of human-caused landcover changes on climate are at least as important and quite possibly more important than those of carbon dioxide.

"Through landcover changes over the last 300 years, we may have already altered the climate more than would occur with the radiative effect of a doubling of carbon dioxide."

If carbon dioxide (CO2) emissions continue at current rates, atmospheric CO2 concentrations are expected to double by 2050.

Land use changes matter because different types of surface strongly affect the atmospheric distribution of the Sun's energy.

The study offers several examples:

• replacing a rainforest with crops means less transpiration (evaporation of water from the leaves), which means warmer temperatures
• irrigating farmland means more transpiration, and greater evaporation from moist soils, which cools and moistens the atmosphere and can alter precipitation and cloudiness
• planting or replacing trees in forest regions with heavy snowfall means less reflected sunlight and more heat absorption - a net warming effect despite the growing trees' use of CO2
• reforestation can increase local transpiration, increasing the water vapour in the air - water vapour is the biggest contributor to greenhouse gas warming.

At sea, periodic El Nino disturbances in the Pacific create moist rising air and thunderstorms, which can affect the weather thousands of kilometres away.

Rethinking the effects

The researchers say: "Tropical land surface changes should be expected to play a greater role on global climate than El Nino, given that thunderstorms prefer to form over land, and the fact that the large area of tropical land-use changes far exceeds the relatively small area of water responsible for El Nino.

Rainforest loss means more warmth

"Impacts of land use changes are harder to detect because they are permanent, as opposed to El Nino, which comes and goes."

They propose a new way to measure the impacts of both greenhouse gases and landcover changes - a formula that quantifies all the anthropogenic climate change factors in terms of the amount of heat redistributed from one area to another.

This heat redistribution is stated in terms of watts per metre squared. So if a torch generated heat of one watt covering a square metre, the heat energy emitted would be one watt per metre squared.

The researchers hope in this way to achieve a more accurate picture of all of humanity's influences on the climate.