Climate change: Heat waves become more intense

World leaders have been meeting for 29 years to try to curb global warming, and in that time Earth has become a much hotter and deadlier planet.

Statistics show trillions of tons of ice have disappeared over that period, the burning of fossil fuels has spewed billions of tons of heat-trapping gases into the air, and hundreds of thousands of people have died from heat and other weather disasters stoked by climate change.

More than 100 world leaders descended on Rio de Janeiro in 1992 for an Earth Summit to discuss global warming and other environmental issues.

That set up the process of international climate negotiations that culminated in the 2015 Paris accord and resumed Sunday in Glasgow, Scotland, where leaders will try to ramp up efforts to cut carbon pollution.

Earth has warmed more in the last 29 years than in the previous 110. Since 1992, the world has broken the annual global high temperature record eight times.

The yearly global temperature has increased almost 1.1 degrees Fahrenheit (0.6 degrees Celsius) since 1992, based on multi-year averaging, according to the U.S. National Oceanic and Atmospheric Administration.

Climate scientist Gerald Meehl says even a small increase in the global average affects temperature extremes.

"We're twice as likely to set a record high maximum than a record low minimum," said Meehl, a senior scientist at the National Center for Atmospheric Research. "That's kind of what climate change is, it's kind of shifting the odds towards certain outcomes."

In the early 2000s, Meehl and his colleagues predicted that heat waves in the 21st century would hit normally temperate regions that had not yet adapted for extreme heat events.

"We looked in the future in a warming climate with increasing greenhouse gases, and we saw these changes in the patterns of future heat waves," Meehl said.

That's exactly what happened in the Pacific Northwest in June.

"It's been estimated that there were over 600 excess heat related deaths in Washington and Oregon alone in one week," said Dr. Renee Salas of Massachusetts General Hospital in Boston.

Salas says heat is one of the most visible ways climate change affects health.

"Extreme heat can cause damage to the brain, worsen mental health impacts," she said. "It can affect heart and lung disease, kidney disease."

And people who are more exposed to heat and less able to adapt are the most vulnerable to those effects.

That includes children and the elderly, who may be more susceptible, as well as workers who frequently labor in the sun and families without air conditioning or access to green spaces.

"In many areas that are low wealth, those communities may not have trees," said Sacoby Wilson, an environmental health scientist at the Univeristy of Maryland, College Park. "Those committees may have older buildings, so they may not have air conditioning, and those communities may have a lot of concrete, a lot of asphalt."

Both materials absorb then release heat. Trees and other greenery bring area temperatures down.

Unless top-polluting countries reverse course, heat waves are likely to become more frequent and intense.

"Those kinds of changes are ones that we kind of expected to happen and they have happened," Meehl said. "And they're going to continue to get worse as we go forward."