Thursday, February 26, 2015

It’s freezing out there! – So much for global warming?

by Ben Brown-Steiner, Ph.D.

            Yikes it’s cold out there! This winter has brought record cold temperatures to Ithaca [1,2] with continuing waves of Arctic air [3] making life pretty uncomfortable. All the Finger Lakes are frozen except for Cayuga and Seneca [4] but with temperatures expected to barely top freezing for the foreseeable future we may yet see a frozen Cayuga Lake for the first time since 1978-1979 [2].

            The past several weeks are undeniably colder than normal, but what’s normal? Is this simply cold weather or does it imply a cold climate? What can we say about how unusual this winter is, and does it have anything to do with climate change? To answer these questions, let’s conduct some rudimentary data exploration. This is the first step when trying to understand scientific data.

           Expanding the analysis done in a previous post, here are the 10-year average maximum, average, and minimum daily temperatures (˚F) for this December, January, and February (so far). The purple line marks the climatological long-term average:

Ithaca 10-Year Average Temperature (˚F) from [5]

            Looking at the long-term climate we can expect winter temperatures to drop from 30˚F in December to a bottom of around 20˚F in January and then a slight increase to around 25˚F by the end of February. This 10-year running average matches this climatological trend with some noise (a ten year average of weather is not yet climate so we expect this noise).

            Compared to these 10-year averages, how does this year’s December-January-February compare?

Ithaca 2015 Daily Temperature (˚F) from [5]

            Yikes! The average climatological January is 28˚F while this January averaged 17˚F. The average climatological February is 26˚F with this February averaging only 11˚F. This previous December, however, was slightly warm: the average December is 28˚F while this December averaged 32˚F. Taken as a whole, however, this is indeed an unusually cold winter.

            Besides the cold temperatures, are there other aspects of this winter that are unusual? Is there more or less variability? Do other years show similar anomalies, either low or high? Here’s an animated comparison of the past 10 Ithaca winters:

Animation of Daily Temperatures (˚F) for Ithaca from the Past 10 Years from [5]

            Take a look and see if you can find anything unusual. Check out this previous post for good ways of examining these types of data.

            It’s difficult to look at the current weather and draw conclusions about climate, so let’s look at these data from different perspectives to see what we can see. My goal here is to tune your baloney detectors when being presented with weather and climate data and to do some data exploration. Data itself can’t lie, but certain interpretations or presentations of the underlying data can lie, especially if it is being presented out of context. Always be skeptical when you’re presented with data! If portions of data or methods are being hidden, there’s likely a hidden agenda in the presentation.

            For example, here is the average February temperature for the past four years. Based solely on this graph, what can you conclude about Ithaca winters?

Average February Temperature (˚F) for Ithaca from 2012 – 2015 from [5]

            I can hear people shouting in the background: “Look! No such thing as global warming!” And sure, based only on the average temperature of the past four Februaries, completely removed from any larger time or regional context, that might make sense. But what are we missing when we leave out the context? Let’s zoom out to the average temperature of the past ten Februaries:

Average February Temperature (˚F) for Ithaca from 2006 – 2015 from [5]

            Looking at ten years of data we can see that there isn’t much of a trend. The fact that the past four Februaries line up in a nice straight decreasing trend seems to be more of a coincidence than a statement about climate.

            Let’s zoom out a little more. What’s the past ten years worth of December-January-February averages look like?

Average December-January-February Temperature (˚F) for Ithaca from 2006 – 2015 from [5]

            The unusual February anomaly in Ithaca is much less apparent here. There appears to be a slight decrease in the winter temperature in the past ten years, but it’s not really a robust trend (or in more technical speak, it doesn’t seem to be statistically significantly different from no trend at all). Let’s zoom out some more. What do the previous 100 winters look like in Ithaca?

 Average Winter Temperature (˚F) for Ithaca from 1900 – 2015 from [6]

(NOTE: The data in in this figure is taken at a different site than the data from the previous figure, thus the Ithaca winter temperatures are not an exact match)

            We definitely get a different perspective here. The year 2012 was the warmest winter of the decade, so it’s not an ideal place to start looking at temperature trends. What we do see is some slow (over multiple decades) increases and decreases, with a maximum in 1932, after which winter temperatures generally decreased until 1978, after which they increased again until around 2000, after which we see no real trend in winter temperatures. These changes, however are small compared to the “noisiness” of the data.

            This 100-year perspective puts the 10-year perspective into a broader climate context. Among some slow variations in temperature from decade-do-decade we see a lot of year-to-year variability. By looking only at 10 years of data we’re missing a lot and we have to be careful about what we conclude from only 10 years of data if we’re talking about climate.

            While we’re looking at the previous 100 years of Ithaca winters, have you ever heard someone older than you talking about how the weather or climate was clearly different when they were young? Did you trust their interpretation and their memory? Let’s take a look. Here’s the average winter temperature for each decade during the past 100 years:

Average Ithaca Winter Temperature (˚F) for Each Decade from 1900 – 2015 from [6]

            If they were born in the 1950’s, 1960’s, or 1970’s, then it was indeed colder when they were young, although only by 3 – 4 ˚F. If they were born in the first half of the century, then these current temperatures are close to what they remember when they were young. Looking at the century as a whole, temperatures have certainly fluctuated but there is no evidence of a clear, unambiguous trend.

            What if Ithaca is unique? What do the same data look like for the greater Northeastern region? Here’s the December-January-February temperature trend for the entire Northeast for the past 100 years:

Average Winter Temperature Anomaly (˚F) for the Northeastern US Region from 1900 – 2011 from [7]

            Here we see that there is a moderate trend showing increasing temperatures. Over the past 100 years, winters in the Northeast have warmed by around 2˚F, although there’s a lot of variability in this data (i.e. the noise is large compared to the signal). Ithaca’s trends don’t quite match this regional trend, but Ithaca is just a single city in this broad region.

In Conclusion

            This broader climate context deepens our understanding of Ithaca’s unusual winter weather we’re experiencing this year and provides us a long-term perspective. Looking at the data like this is usually the first step in trying to understand a scientific phenomenon since it helps to understand the larger picture. Jumping right into a smaller portion of the data and drawing conclusions (e.g. the previous four Februaries) leads to distortions and misunderstanding, and we all need to be wary of data that is presented without this greater context.

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