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If the World is Warming, Why am I so Cold?

December 18, 2013

This year in particular, December seems to have gotten off to a cold start in the Valley. The thermometer on our monitoring station at Sky Mountain recorded a low of -15 degrees Fahrenheit in the first week of the month. The thermometer at my own house, mid-valley, reads in the low teens each morning. So why, if global atmospheric warming is really happening, do these bitterly cold days still occur? To answer this question, you first have to take a look at the difference between weather and climate.

Humans tend to think in terms of weather. Might it be cold today? Is there enough snow for good skiing this week? Will there be water for my crops this summer? All of these weather conditions--measured as temperature, inches of rain, or wind speed--vary widely, sometimes even within the same day. Climate is more stodgy than weather. It is slower to change and describes the average conditions found in a region over an extended period of time: decades or more. Climate describes what conditions generally existed in the past and allows us to project what conditions are generally likely occur in the future.

Weather, by contrast, describes what is happening at a single given moment. Ben Kirtman, a meteorologist who spoke on this topic at a public lecture in Aspen in 2011, described weather as being "the statistics of climate."

Think for a moment about baseball. To a certain degree, baseball players, like climate, are rated in terms of their averages. You might have an excellent player experience an off-day and not hit a single ball thrown at him. Conversely, you might have a player who usually strikes out hit a home run one game. People trying to predict the outcomes of future games are not likely to base their predictions on the single off-day or a lucky home run. They want to know what the players' averages are. Climate, likewise, is not determined by an outlier; it is determined by overall trends.

For example, let's take a look at an indicator of weather conditions in Aspen for 2011 vs. 2012. The indicator we will use is frost free days, or the number of days between the last frost in Spring and the first frost in Fall. The more frost free days in a year, the longer the warm season. In 2011, Aspen had 128 frost free days. In 2012, that number dropped to 106. If you looked only at these two years, it would seem as though frost free days in Aspen are decreasing, in other words, it is getting colder, and the growing season is not lasting as long. Looking only at these two years, however, would be like watching your baseball player for just two innings. When you place these two years in perspective with 70 years worth of data, frost free days tell a very different story. Below is a graph showing the average number of frost free days for each decade since 1940 in Aspen, Colorado.

Looking at average frost free days over this longer period of time, you see an upward trend. Compared to the 1940's and 1950's, this decade has an average of 30 more frost free days--an entire additional month that did not have frost.

Frost Free Days from the Aspen Weather Station. Note that the station moved to a higher elevation in 1980. Data available through NCDC. For further information on ecology and frost free days, visit the FHI website.

Looking at extreme low temperatures over the same time period tells a similar story. Some individual years have a large number of days below 0 degrees Fahrenheit, while others have very few. Looking at the number of days per year below 0 over the long term reveals a sharp downward trend from 1940 to 1980 and a less steep slope from 1980 to 2012. In other words, very cold days are becoming less common.

Days below 0 Deg F. Data is from the Aspen weather station, which moved to a higher elevation in 1980. Data is available through NCDC. For further information on ecological importance of very cold days, visit the FHI website.

Trends matter in nature. As was discussed in November's post, living things have a range of flexibility for conditions in which they can survive. If that range is exceeded or the change persists for too long, then the living thing must adapt or perish. For example, the bark beetle that wreaked havoc on Colorado's Ponderosa pines a few years ago was able to proliferate so successfully because several years followed one another where the temperature did not remain below 20 degrees F for long enough to disrupt the beetle's life cycle. As a result, the beetles thrived, placing pressure on pines. The spruce bark beetle, has a similar life cycle, where overwinter survival is determined by temperature. If the warmer winters continue, then the pines and spruce of the Rocky Mountains will be increasingly forced to adapt or may may dies off, leading to a shift in which trees dominate this regions.

Frosty days and icy nights have been a part of Colorado's climate for 100's of years before today, and the ecosystems and native species that make the Roaring Fork Valley unique are present because of their ability to succeed in those previous conditions. Changes in weather can determine the survival of an individual, but changes in climate can determine the survival of an entire species.