THE BLOG
06/19/2013 12:09 pm ET Updated Aug 19, 2013

Colorado Wildfires Are Linked to Global Warming

Last week I was able to witness firsthand one of the frightening impacts of climate change on the United States as a wildfire spread across part of the Rocky Mountain National Park.

I was attending a special conference in Granby, Colorado, which was organized by the American Geophysical Union to explore climate change communication.

The afternoon of June 11 had been set aside for a field trip to the nearby national park at the northern end of the state.

As the conference coach approached the park entrance, we could see smoke billowing from behind a forested ridge. It was a fire caused by a lightning strike the day before, and which had flared up in high winds.

The Big Meadows fire has already engulfed more than 600 acres of forest and could rage for several more days.

So what is the link to climate change?

The wildfire is being fuelled by a great abundance of dead trees that have fallen victim to the voracious appetite of the mountain pine beetle, Dendroctonus ponderosae.

The beetle lays its eggs in the bark of pine trees, and the larvae burrow inside, satisfying their appetites by eating the inner parts of the tree. However, the lifecycle can only be completed if the larvae are eventually released from the confines of the wood, so adults launch mass attacks to lay thousands of eggs in a susceptible tree within just a few days in order to kill it.

A pine tree can release thick sap to try to repel the larvae, but if it is overwhelmed by the sheer number, its inside is devoured and it eventually becomes unable to transport water and nutrients along its core, leading to a slow death. The characteristic green needles turn red and may persist for a few years before dropping off to leave behind a dried up corpse of ash grey trunk and branches.

The mountain pine and other bark beetle species have reached epidemic proportions across western North America. The current outbreak in the Rocky Mountain National Park began in 1996 and has caused the destruction of millions of acres of ponderosa and lodgepole pine trees. According to an annual assessment by the state's forest service, 264,000 acres of trees in Colorado were infested by the mountain pine beetle at the beginning of 2013. This was much smaller than the 1.15 million acres that were affected in 2008 because the beetle has already killed off most of the vulnerable trees.

Researchers have discovered that the lifecycle of the mountain pine beetle is controlled by temperature. Adults emerge at the same time, triggered by the first onset of warm weather. Bouts of cold winter weather kill off many larvae, acting as a natural limit on the beetle population.

But western North America has been experiencing milder winters and warmer summers over the past few decades due to global warming, so the beetle population has exploded in Colorado and elsewhere. And the warmer temperatures have even allowed the beetle to evolve so that it completes two lifecycles in a year, instead of just one.

Furthermore, pine trees become more susceptible to beetle attacks if they are weakened by a shortage of water. Colorado, like much of North America, has experienced drought conditions for much of the past two years.

It is little wonder then that the draft published in January of the new United States National Climate Assessment concluded:

"Bark beetles have infested extensive areas of the western U.S. and Canada, killing stands of temperate and boreal conifer forest across areas greater than any other outbreak in the last 125 years...Climate change has been a major causal factor, with higher temperatures allowing more beetles to survive winter, complete two life cycles in a season rather than one, and to move to higher elevations and latitudes."

The report also warned that "the dead trees left behind by bark beetles make crown fires more likely".

So climate change really is making wildfire risk worse in Colorado and across western North America. But while the Big Meadows fire may not pose much of a threat to citizens or their property, another one further south has just become the most destructive wildfire in Colorado's history.

The Black Forest wildfire started near Colorado Springs on 11 June and has already caused two deaths, destroyed nearly 500 buildings and prompted the evacuation of 38,000 people from 13,000 homes.

Last year's Waldo Canyon wildfire had been the most damaging, destroying 347 buildings and killing two people.

Although the cause of the Black Forest fire has not yet been established, it is burning in an area heavily forested by the ponderosa pine, another favourite meal of the mountain pine beetle. Could climate change be responsible for this disaster as well?

Bob Ward is policy and communications director at the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science.