Putting Short-Term Warming Trends in Context
Monday, October 18, 2010 at 5:19 pm
In keeping with my pledge to write about climate science more often, I thought I’d share the latest climate data from the National Oceanic and Atmospheric Administration.
According to NOAA, the arm of the federal government that monitors climate:
The first nine months of 2010 tied with the same period in 1998 for the warmest combined land and ocean surface temperature on record. The global average land surface temperature for January-September was the second warmest on record, behind 2007. The global ocean surface temperature for January–September was also the second warmest on record, behind 1998.
But what does this mean exactly and what do these numbers tell us about climate change?
I put this question to NOAA scientist Jay Lawrimore, chief of the climate analysis branch at NOAA. Though he warns that it’s difficult to extrapolate based on short-term climate data, Lawrimore says that the numbers play into the long-term warming trend. In fact, he says global temperatures should actually be cooling during the months when La Nina cools ocean temperatures in the Pacific Ocean. Given that temperatures increased despite La Nina, Lawrimore says, “The fact that it was warmer than average speaks to the fact that the overall trend in global temperature is a warming trend.”
Here’s a transcript of our conversation, edited for clarity:
How should people see these new data, and what do they represent in the larger climate narrative?
I think this represents the role of natural variability and the role of long-term trends in climate. And the fact that these play a key role in temperatures from month to month. Specifically, I’m referring to the presence of La Nina and the cooling influence that La Nina has on global temperatures. That’s kind of the natural component that I’m referring to and the fact that we have this long-term trend toward warmer temperatures, but there are things that come into play on a month-to-month and a seasonal basis that can affect global temperatures. While the trend may be positive, it doesn’t mean that every month is going to be warmer than the preceding month, or every September is going to be warmer than the previous September or the September before that.
Can you explain exactly what La Nina is in basic terms?
It is a cooling of ocean temperatures in the equatorial Pacific. These cooler ocean temperatures, they have an influence not only on the global average temperature, but on overall weather patterns. They change the location of convection in the equatorial regions of the Pacific, which then has dampening influences in the way that atmospheric patterns move and weather systems move. And so, with La Nina, in general we have generally wetter conditions in the Pacific Northwest and generally drier conditions in the Southern tier. And also on a global scale with cooler temperatures on the Pacific, there’s less heat being put into the atmosphere and it tends to dampen the overall global temperature.
When you say dampened, what do you mean?
Lower than the global temperature would have otherwise been when La Nina wasn’t present.
But the data showed that the temperatures were higher?
It was still warmer than average and the fact that it was warmer than average speaks to the fact that the overall trend in global temperature is a warming trend. Colder temperatures are rising and long-term scientists broadly believe this rising temperature to be the result of increased greenhouse gas concentrations in the atmosphere. So you have this continual increase in greenhouse gas concentrations in the atmosphere that has a warming influence on the global temperature. But, like I said before, each year is not necessarily going to be warmer than the year before because there are other natural variations of the climate that influence the global temperature. So, the long-term trend is up, but some years may be a little cooler than others.
So, this information sort of plays into the long-term trend, but any individual temperature or group of temperatures within a few months is just one part of the longer-term story?
Yes, when you’re looking at climate change you really want to be looking at long-term changes. You don’t want to be looking at any one in particular month or season or year. That in itself doesn’t give you much information as to the change over the long term that is really important in understanding how greenhouse gas concentrations are influencing the climate.
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