Not So Simple
"These things are consistent with the kind of changes we expect to see [from global warming]," said Kathleen Miller, a NCAR scientist who studies natural resource systems and the impact of climate change on societies. But she was wary of identifying global warming's role.
"Anything you see is some combination of natural internal variability and the effects of climate change," she said. "For any particular event, you can't clearly separate out what is the primary influence."
"We're more certain of some things than others," said Meehl. "Temperature is the one where there's the greatest certainty."
For instance, the United States had twice as many record-high daily temperatures than record lows from 2000-2009 compared to the 1950s, when the two were about equal, according to a study by Meehl.
That same trend is seen in decadal average temperatures, said Washington of NCAR, which "have gone up over time." Other statistics show "the number of heat waves is increasing and the number of cold waves is decreasing," he said.
Hurricanes, which get their power from hot humid air over the oceans, are also on the rise in parts of the world. Trenberth says there is a direct causal link with extra atmospheric heat.
"It's now 1 degree Fahrenheit warmer and there's four percent more moisture [over the oceans] than 30 to 40 years ago. That's the environment in which all storms now develop ... [and these are] conditions that tend to make storms more intense."
In the tropical North Atlantic, said Kharecha, there's a "strong correlation" between increasing sea surface temperatures and the "frequency and intensity" of hurricanes since the 1950s. But that trend cannot be found in hurricane trends worldwide, and the lack of reliable data before the 1960s — when satellites were first put into use — means more research is needed.
Extremes in rainfall patterns are also likely being affected, data show. In the U.S., the amount of heavy precipitation events increased 16-20 percent from 1958 to 2007, said Washington, referencing a 2009 report from the U.S. Global Change Research Program.
"The hydrological cycle of the earth is spinning up as we put more energy into the climate system," said Kiehl.
A Key Metric
With global average temperature expected to rise another 3.6 to 7.2 degrees Fahrenheit over the course of the current century, scientific certainty over just how the extra energy will ripple through the climate system will likely lag behind the occurrence of extreme weather events.
But there remains little doubt among scientists that a planetary energy imbalance is changing the weather system. This, more than rising average temperatures, is perhaps the most important metric for understanding changes to the climate system.
"Temperature is a way to measure the heat, it's a great metric," Kharecha said. But energy imbalance is "the most fundamental gauge of the state of the climate system at any given time" and can help provide insight into "how much we must reduce atmospheric greenhouse gas levels to restore the planet's energy balance."