It's Been So Cold Lately!

Marc & Jetta discuss the weather:

J.: The weather here is so strange! It's so cold! And the tornadoes have been worse than ever this year! Isn't all that connected to Global Warming?

M.: I don't blame Global Warming for the cold weather. I blame La Niña instead.

Sacramento temperatures were below average almost the entire month of May. Yesterday, it was 14 degrees F below average!

Anytime you have strong, sustained onshore flow in this part of the world, you get temperatures that are below-average, because the Pacific Ocean is cold. Plus, we've had a sustained trough in the West: the trough of a long-wavelength, stationary Rossby wave. That helps bring in colder air from the polar regions. So, it's been cold here.

This year is a lot like last year, when it was colder-than-average too. And for the last two years, we've had one of the strongest La Niña episodes ever. There's a connection there! This La Niña ended with an abrupt bang less than a month ago, so things will change again, but not enough time has passed for the collapse of La Niña to have an effect here yet. The weather is still acting as if La Niña is going full-tilt.

The climate forecasters saw and understood the broad impact of La Niña last year, and it's played out almost like a textbook case. The only surprise was that California didn't get engulfed in drought too. We were lucky: the drought extended across the entire Southern tier of states, except California.

Here's a recent graph of the Southern Oscillation Index (SOI), demonstrating the recent, abrupt end to La Niña:

J.: But isn't all that associated with Global Warming?

M.: Not really. La Niña would have occurred whether there was Global Warming, or not. There might be loose connections between the two phenomenon, though.

J.: So what is the effect of Global Warming?

M.: The effect of Global Warming is to make the Earth warmer.

J.: On TV, they said the recent tornadoes might be connected to Global Warming.

M.: Well, we've always had tornadoes. In general, they are small in size, and their strength depends on multiple factors. Some people do make connections, however:

"Basically, as we warm the world up, the atmosphere can hold more moisture in it," said Anne Jefferson, an assistant professor in the geography and Earth science department at the University of North Carolina at Charlotte.

"Weather patterns that used to be limited to the South move farther north now," she said. "Both of those things together will increase the frequency with which we see these big rainstorms, and those are likely to increase flooding in the future."

But it's also important to remember that since Global Warming is forecast to have a greater effect in polar regions than equatorial regions, it might also reduce the strength of storms, by helping erase the temperature difference between the equator and the poles. So, the ultimate effect of Global Warming might be that the continents will tend to dry out and desertify, but where it does happen to rain, it will rain heavier. But it's probably unrelated to why it's so cold here, now.

J.: Are you saying that Global Warming has no effect?

M.: No. The effect of Global Warming is to make the Earth warmer. That is what it does.

J.: But they say it will cause flooding and disaster, and that the changes are coming faster-than-expected!

M.: Yes, it might do exactly that. The glaciers and ice caps are melting faster-than-expected, and all that.

You can look at weather data from just about anywhere and see the effect of warming. The effect is most-visible in morning low temperatures in dry locations (where water vapor has a less-complicated and less-confusing effect overall).

For example, I looked at morning low temperatures for April for Mina, Nevada (a dry month in a dry place), and starting in the mid-1970's you can see these low temperatures rising. No matter where you look on Earth, you tend to see similar trends.

Remote deserts are nice, because the greenhouse warming associated with water vapor is minimized in the deserts, so if you see any changes, it's likely carbon dioxide to blame, rather than any changes associated with the urban heat island effect, or changes in roughness associated with changes in land use, or first-order changes in water vapor. And minimum temperatures are where you would spot the change first, because turbulence is at a minimum for these mostly-stagnant conditions. Cooling occurs principally by radiation.

Using US HCN data for Mina, NV (on the road between Hawthorne and Tonopah), and looking at just the monthly mean minimum temperature for the month of April (a rather dry month), it's easy to spot the warming signal. And this particular data set ends in 1994, so recent warming isn't even on here.

J.: Starting in the mid-1970's?

M.: Yes, Global Warming started happening some time ago. People have been talking about it since the 1880's.

J.: Some people say it's irreversible!

M.: I'm sure it is irreversible. But neither of us will live long enough to see the full impact of Global Warming.

J.: So, that's something!

M.: Yes, that's something, all right, but it's probably unrelated as to why it's cold, here, now. Like I say, that's probably La Niña at work.