It's quite cold across much of the United States right now, thanks to the dread "polar vortex." Bitterly cold. Horrifically cold!

So what does this tell us about global warming? Not very much. Sorry. A single cold snap in the U.S. doesn't disprove global warming any more than the record heat waves currently hitting Australia prove that it's happening. But since a lot of people — like Donald Trump — seem confused on this point, it's worth recapping a few basics:

1) Global warming refers to the whole planet, not just the United States. The term "global warming" typically refers to the rise in the average temperature of Earth's climate system since the late 19th century, as well as predictions for future warming. A key bit there is "Earth's average temperature." It can be very cold in one part of the world and very hot in another at the exact same time. (Sometimes the exact same weather event can do both: The jet stream is currently making some parts of the U.S. unusually hot and some parts unusually cold.)

What we're interested in is whether the global average is changing over a longer period. That's impossible to judge from a single point in time in a small area — the continental United States is less than 2 percent of the Earth's surface.

2) For example: December 2013 was an unusually warm month even though it was colder in the U.S. So let's take this past December as an example. North America was colder than the average over the past decade. But Europe and Russia were much hotter than average. India was cooler than average. Australia was warmer than average. And so on:

What happens when you add it all up? Early data suggests that December 2013 was tied for the 2nd-hottest December on record since 1979, the beginning of satellite measurements, according to data from the University of Alabama-Huntsville. Meanwhile, global average temperatures for all of 2013 are expected to be among the 10 highest since 1850 (though we still don't have a final count yet).

So you couldn't really infer all that much from a cool month in the United States.

3) The global temperature won't necessarily go up every year. Focus on long-term trends. Sort of a corollary to #1 and #2. This is a good chart to watch:

The global average surface temperature has clearly gone up since the 19th century, by more than half a degree Celsius. But there's also fair bit of variation year to year.

Climate scientists are quite sure that if we keep adding carbon dioxide to the atmosphere, that will trap more heat at the Earth's surface and the global average temperature will continue to rise over time. But carbon dioxide isn't the only force affecting Earth's climate. There are El Niño and La Niña cycles, which can shift heat into and out of the ocean. There are volcanoes. There's air pollution. There are changes in solar activity. And so forth.

Scientists are currently debating which of those other factors might be responsible for the slower pace of surface warming since 1998. And the Intergovernmental Panel on Climate Change (IPCC) expects that these natural fluctuations will continue to be significant until about mid-century. But in the long run, the IPCC says, global average temperatures should trend upward with an increase in greenhouse gases.

4) Global warming isn't expected to abolish winters in the U.S. anytime soon. Right now, climate experts are worried about a 2°C to 4°C rise in global average temperatures by the end of the century. That would create all sorts of disruptive changes. But those few degrees aren't enough to completely undo the larger swings in temperature we see each year between summer and winter in many parts of the world.

Indeed, many climate models suggest that we'll still see record cold snaps in the United States as the planet heats up. They'll just become much less frequent over time — while record heat waves will become increasingly common. See this paper in Geophysical Research Letters from 2009: Over the past decade, it notes, the U.S. has experienced about two daily record high temperatures for every record low. If the planet keeps heating up, that ratio will shift to 20:1 by mid-century. There will still be record lows in many areas. They'll just be rarer. Like so:

5) Heavy snowstorms will also still be possible as the planet warms. This sounds bizarre, but it makes some intuitive sense. As seen above, global warming isn't going to eradicate winter temperatures in the United States anytime soon. But a warmer planet will allow the air to hold more moisture on average. So, in theory, you could have the ingredients for more intense winter storms.

Will they still be as frequent? That's less clear. One 2006 study found an increase in winter storm activity in the Midwest and Northeastern United States over the past century, as the Earth has warmed. And the IPCC says that heavy precipitation events in the Northern Hemisphere are expected to increase as the planet heats up. But that prediction is for all seasons, not just winter, and there's less certainty on more fine-grained forecasts.

6) Yes, there is a theory for how global warming could cause severe cold in the U.S. — but it's still heavily debated. Right now, the Arctic region is warming rapidly. And a few scientists think this could cause the jet stream to slow down and weaken and meander all over the place more often.



(The Washington Post)

That could have lots of unpredictable effects. It might cause storms or heat waves to linger in one place for longer periods of time. Or it could allow bigger blasts of frigid Arctic air to travel down to the United States — as is happening right now.

But key caveat: This is a relatively new idea, and there's still a whole lot of debate over the link between Arctic warming and extreme weather. Jennifer Francis of Rutgers sketched out the theory here. In August, Elizabeth Barnes of Colorado State disputed the link (and Francis responded here). No doubt there will be a lot more research done.

For now, the consensus view still holds that global warming will bring fewer cold snaps to places like the U.S., not more. The IPCC in 2007 predicted that there was "likely to be a decline in the frequency of cold air outbreaks... in [northern hemisphere] winter in most areas."

7) A few points on Antarctic sea ice. Occasionally we'll hear that sea ice in Antarctica has been expanding lately and that's an inconvenient problem for the theory of global warming. This came up recently after a bunch of climate researchers on a ship got themselves stuck in Antarctic sea ice. But it's worth putting this in context.

Note that there are two types of ice in Antarctica. First, there's sea ice, which is the ice floating in the ocean around the continent. For reasons that are still unclear, the extent of Antarctic sea ice has indeed been growing in recent years. This increase is less drastic than the long-term decline of summer sea ice up north in the Arctic, but it's real nonetheless. And it's still a mystery.

But that's not the only thing going on down in Antarctica. There's also land ice. This is the snow and ice that sits on top of land in large ice sheets. And it's arguably more relevant from a practical standpoint, since when that ice melts and falls into the ocean, it pushes up sea levels. (Changes in sea-ice extent, by contrast, don't directly affect sea levels very much — though they can have indirect effects.) And current estimates suggest that Antarctica is losing land ice:

So there you go. It's horribly cold outside. The planet's still warming. Strange but true. Now here's a fun video of how Canadians are entertaining themselves in subzero temperatures.

Further reading: As always, our colleagues at Capital Weather Gang have indispensable coverage of the polar vortex and the current cold weather.