Explainer: How Climate Change Fuels Extreme Heat Worldwide

By Alison Withers and Gloria Dickie

(Reuters) - Climate change is driving dangerous heat waves across the Northern Hemisphere this week and will continue to deliver dangerous weather for decades to come, research shows.

"It is a worldwide heat wave that we are now suffering. That puts the heat under our decisions," said Christiana Figueres, a former U.N. climate agency chief.

Here's how climate change is pushing heat to new extremes.

HOW IS CLIMATE CHANGE DRIVING HEAT?

As the continued burning of fossil fuels releases more carbon emissions to the atmosphere, the air can trap more heat from the sun – causing the average global temperature to rise over time.

Already, the global average temperature has risen nearly 1.3 degrees Celsius (2.3 degrees Fahrenheit) since the start of the Industrial Revolution, when Western countries began burning coal and other fossil fuels.

That higher baseline means climate change is already making all heat waves hotter than they would have been without atmospheric warming. They are also becoming more frequent overall – and more dangerous.

Any significant heat wave "has been made substantially more likely and warmer than it otherwise would have been as a result of human-caused climate change," UCLA climate scientist Daniel Swain told reporters earlier this month.

"At this point, that is an almost trivial statement to make because there's so much evidence supporting it."

HOW MUCH OF A FACTOR IS CLIMATE CHANGE?

Beyond global warming, there are other factors and conditions that can affect heat waves. Climate systems such as El Nino or La Nina can have a big impact, along with regional circulation patterns.

Land cover can also play a role, with dark surfaces and built environments tending to get hotter than reflective white surfaces or than natural systems like forests or wetlands.

To find out exactly how much climate change influenced a specific heat wave, scientists conduct "attribution studies".

They have conducted hundreds of these studies over the last decade by running computer simulations to compare today's weather systems with how they might have behaved if humans had not changed the atmosphere's chemistry over the last century.

For example, scientists with World Weather Attribution have determined that the dangerous heat across South Asia in April was 45 times more likely to have occurred thanks to climate change. During that heat wave, thermometers in the northeast Indian city of Kolkata hit 46 C (115 F) – a full 10 degrees higher than the seasonal average.

WHAT CAN WE EXPECT IN THE NEAR FUTURE?

Even if all carbon emissions were halted today, the world has already emitted enough to ensure that climate change will continue to push temperatures upward for decades.

The world must cut emissions in half from 1995 levels by 2030 – and to net-zero by 2050 – to have a chance of keeping the average global temperature rise to around 1.5 C (2.7 F) above the preindustrial average, according to scientists with the Intergovernmental Panel on Climate Change.

Instead, global emissions have only gone up since 1995. The world is currently on track to reach 2.7 C (4.9 F) by 2100, blowing past the 1.5 C (2.7 C) threshold beyond which scientists predict catastrophic and irreversible climate impacts.

The fact that millions of people "in the United States being subjected to unprecedented heat waves is indicative of the fact that we have yet to address the worst of climate change," Figueres told Reuters on Thursday.

(Editing by Katy Daigle and Josie Kao)