December 2015’s crazy weather




Spring ahead

As I write the temperature in NYC is approaching 80F. Most trees, though, are still leafless.

It’s not unusual to have days like this here around this time of year, when the temperature races ahead of the foliage as the sun rapidly climbs higher in the sky. This year the leaves and flowers are coming in late, making a day like today almost inevitable.

I haven’t been able to find a quantitative analysis online of just how late this year’s leafout is, relative to long-term averages, but the cherry blossoms in Washington, DC seem to have peaked about a week late, and my guess is that we’re running at least that far behind schedule here in New York after the harsh winter we had.

The long-term trend, on the other hand, is towards earlier blooming as the climate warms. But as with everything else about late winter and early spring, you wouldn’t have known that from this year in the northeast.

I went for a walk this afternoon along the Hudson River promenade, and it was a good few degrees cooler down by the water. The transition seasons are when the temperature contrast between land and large bodies of water peaks, as the large thermal inertia of the water remembers the season past while the land responds quickly to the sun.

Back in March, when it seemed like winter would never end, I found it comforting to keep reminding myself that the seasonal cycle is inexorable – nothing stops the earth’s orbit.

Sometimes in climate change debates, you hear someone ask “If meteorologists can’t predict the weather two weeks in advance, how can they predict it 50 or 100 years in advance?” The basic answer is that we aren’t predicting weather far in the future, only climate. The two are different. The seasonal cycle provides a useful way to make this less abstract.

I can’t predict exactly what the weather will be on July 4th, but I can predict with unshakeable confidence that it will be a lot warmer than it was in February or March. In fact, I can predict that summer will be warmer than winter every year, forever. That is a climate forecast. The seasons change because of changes in the radiation incoming at the top of the atmosphere, and that’s exactly the same reason that greenhouse gases are inducing a long-term warming trend.

Sure, there are differences. The summer-winter changes in insolation are much larger than those due to human-induced greenhouse gas changes; the seasonal change is mainly in the visible part of the electromagnetic spectrum while the greenhouse gas forcing is in the infrared; the greenhouse gas influence is global while the seasonal changes are opposite in the two hemispheres; and we have a much longer history of observing the seasonal changes, so a more or less correct prediction can be made empirically, without any physical understanding. But really, the basics are quite similar. So anyone who thinks a climate forecast can’t be done can benefit from thinking about what is behind the changes happening at this time of year.