Any time it’s hot or hotter than average, the subject of global warming invariably arises. The usual question I hear is: Is this global warming (or climate change)? The answer I give is that global warming is not causing hot weather but almost certainly intensifying it.

In other words, think of the greenhouse gases building up in the atmosphere from human activities as steroids. Just like steroids can help a baseball player hit a ball farther and hit more home runs in a season, greenhouse gases are performance enhancers when it comes to hot weather. These gases from fossil fuel combustion and other sources add a little heat to the atmosphere or “juice” it up - loading the dice to make hot weather and hot weather records more likely as long as they continue accumulating in the atmosphere.

Extending the analogy, just like any single homerun can not solely be attributed to a baseball players’ steroid use since it ignores natural ability (and past performance prior to use of performance enhancers), a single hot day can’t be exclusively explained by global warming (as it ignores natural weather variability and past hot days/heat waves; see Andrew Freedman’s column which further explores this analogy).

A small shift in average temperatures can result in a significant number of record heat events (National Center for Atmospheric Research)

“A relatively small shift in the average produces a large change in extremes,” said NCAR scientist Jerry Meehl. “If you shift the whole distribution a little to the right you have more extreme heat. The tail on the far right moves into uncharted territory, breaking new heat records.”

Interested in evidence that this shift towards a hotter climate is real? NCAR supported research recently found record high daily temperatures in the United States are outpacing record low daily temperatures by a ratio of 2 to 1.

This graphic shows the ratio of record daily highs to record daily lows observed at about 1,800 weather stations in the 48 contiguous United States from January 1950 through September 2009. Each bar shows the proportion of record highs (red) to record lows (blue) for each decade. The 1960s and 1970s saw slightly more record daily lows than highs, but in the last 30 years record highs have increasingly predominated, with the ratio now about two-to-one for the 48 states as a whole. (National Center for Atmospheric Research)

UPDATE (4:10 p.m.): CapitalClimate reports through the first week of June, high temperature records have outnumbered cold temperature records in the U.S. by over 10:1.

With greenhouse gases continuing to accumulate in the atmosphere, we should expect heat records to increasingly dominate cold records. Take a conclusion of the Stanford study I referred to yesterday:

According to our projections, large areas of the globe are likely to warm up so quickly that, by the middle of this century, even the coolest summers will be hotter than the hottest summers of the past 50 years.