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Warming 'big threat' to Yosemite
By Matt Walker
Editor, Earth News

Fire buring in Yosemite

Wildfires within California's world famous Yosemite National Park could become more frequent and severe due to climate change, say scientists.

New research in the International Journal of Wildland Fire says warmer temperatures pose a twin threat.

As well as directly triggering fires, they could also melt the snow that covers the forest in winter.

Lightning strikes would then trigger more fires, burning more intensely.

If society continues on its path to increased carbon emissions, places like Yosemite will be inalterably changed. Fires will be more frequent and more severe
Dr Jan van Wagtendonk
US Geological Survey

"People already expect more ignitions from hotter summers," says Dr James Lutz of the University of Washington at Seattle, US, one of the study's authors.

That is because predicted higher temperatures will make vegetation more flammable and allow larger fires to take hold.

"But this research suggests that declines in snowpack will have an additional effect," says Dr Lutz.

He and his colleagues estimate that warmer temperatures will trigger a 20% increase in both the number of fires within Yosemite and also in the area of forest that will burn with a higher severity.

These increased fires will be triggered by lightning strikes.

The reason that will happen is two-fold. First, there is some evidence to suggest that increasing levels of carbon dioxide in the atmosphere lead to more lightning strikes.

After effects of fire in Yosemite
After the burn

Second, and more important in Yosemite, Dr Lutz and his colleagues show that if snowpack cover in Yosemite during the winter falls by 17% by 2050, an amount predicted by conservative climate models, then lightning strikes are more likely to ignite forest fires in the park.

"Yosemite's climate is Mediterranean, with very little summer precipitation and the terrain is mountainous. Accordingly, winter precipitation drives landscape flammability," says Dr Lutz.

His team examined the relationship between snowpack and the ignition and size of fires in Yosemite between 1984 and 2005.

During this period, 1,870 fires burned over 77,000 ha (190,000 acres).

But when snowpack cover decreased in certain years, lightning-ignited fires increased exponentially.

Using these figures, they then extrapolated what would happen if snowpack levels fell by 17% in Yosemite, as predicted.

Smouldering fire In Yosemite

Using 23 years' worth of data acquired by the Landsat Thematic Mapper instrument, the researchers also confirmed that as the area of land that burns increased, the severity of the fire within also went up, causing yet more damage.

Overall, the number of lightning-ignited fires is predicted to increase by 19.1% from 2020 through to 2049, while the area that will burn at high severity each year will increase by 21.9%.

The projections produced by Lutz's team depend on warming continuing according to what is known as the B1 emissions scenario.

In 2000, the International Panel on Climate Change (IPCC), which evaluates and summarises the work of thousands of scientists, published a number of scenarios detailing how much greenhouse gases would be emitted as countries develop.

The B1 emissions scenario assumes a low level of emissions, with carbon dioxide emissions increasing slightly in coming decades but then falling to lower than current levels by 2100.

However, even this optimistic emissions scenario results in global temperatures increasing by 1 or 2°C.

The A1 scenarios assume much more greenhouse gas would be emitted, and temperatures would rise further.

IPCC scenarios for carbon dioxide emissions per year
IPCC scenarios for carbon dioxide emissions per year

"Snowpack declines projected for the A1f1 emissions scenario were extreme," says Dr Lutz.

In the recent meteorological record, Yosemite has always had more snow than predicted by such a scenario.

And since 2000, greenhouse gases have been emitted at an even greater rate than the worst A1fi scenario predicted by the IPCC.

"The current emissions trend implies a snowpack so low that we cannot accurately project how much the fire regime will change," says Dr Lutz.

Yet the authors of the current study do not expect the outcome to be good.

"If society continues on its path to increased carbon emissions, places like Yosemite will be inalterably changed. Fires will be more frequent and more severe," says co-author Dr Jan van Wagtendonk of the US Geological Survey's Yosemite Field Station in El Portal, California.

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