Friday, October 24, 2014

What's the climate like outside today?

By Ben Brown-Steiner

Winter is coming. We all know this. But let’s say that I didn’t know this. What could you do to convince me that winter is coming? If we went for a walk outside what things could you use as evidence?

You could use the colors of the leaves and a description of the seasonal cycle of trees. You could point out the frantic behavior of squirrels and an explanation of what they are burying and why. Both of those things are individual pieces of evidence and a logical explanation of how they fit into a larger pattern, and as such they are pretty convincing.

If it was a cold day you could talk about the decreasing temperatures and talk about how you wished it was still August. That would be pretty convincing. But what if it was an unusually warm day? You would be forced to tell me to forget about the day’s weather and focus on the longer trend. You would tell me to ignore what appears to be counter evidence for your claim. If I were skeptical that winter was actually coming, I may not believe you.

Looking for evidence in an individual day for a change in season, especially if you are talking about temperatures, is really tricky because all we can experience when we step outside is the weather. If I wanted to know what the weather was today you could respond like Calvin’s mother does to his question:
Weather is the day-to-day, hour-to-hour, minute-to-minute fluctuation of the atmosphere we all live in, which is chaotic and highly variable. In order to perceive a seasonal change we need to pay attention to the moving average of daily temperatures over the span of weeks or months. This is because ultimately a change in seasons is not weather. It’s a change in the long-term average temperatures that follows an annual cycle dictated by the tilt of our planet. This cycle is so consistent that during certain times of the year we all expect to see evidence of the change in seasons and therefore we perceive changes in temperatures as evidence of a change in season. During the autumn, it is very easy to experience a cold day and feel that winter is near, or experience a warm day and lament the passing of the summer. But our day-to-day experiences are moments in the constantly fluctuating and highly variable weather. On their own they are not evidence of a change in seasons.

To understand the seasonal cycle, or to understand any long-term average, we have to rely on observations, data, statistics, and pattern recognition. This is especially true if we’re trying to understand the climate. This is because, even more so than the seasons, climate is a long-term average (years to decades) of the weather and averaging is a statistical tool that is abstracted from the weather that we experience every day. For instance, if the high today is 77°F and the low tonight is 35°F, the 24-hour average temperature is around 56°F, which is a poor representation of the hourly temperatures that we actually feel on that day. Even an average of temperatures over a single day is abstracted from real-world experience and is about as useful as that broken clock that is right twice a day.

To get an intuitive feeling for how abstract climate really is, let’s look at Decembers in Ithaca over the past ten years. The following figure is the daily maximum temperature (red), minimum temperature (blue) and average temperature (green) for December 2013.

December of 2013 felt like a weird one. It dropped below 0°F on December 17th only to reach above 65°F on the 23rd. What can we predict about this December based on last December? The unfortunate answer is: not much. It would be foolish to predict that this December would match any of the specific highs and lows from last December. Weather is highly variable. What if we look at the average climate for Ithaca in December? The following figure is the same as the one above but with the climate average for each day in a purple line.

The December climatology shows that, on average, the mean temperature drops from 35°F on December 1st to 25°F by December 31st. Interestingly, last December looks nothing like the climatology. Why is that? How can we say that we would expect any given December to be like the December climatology when last December was nowhere near the climatology? Was last December an unusual December? We can’t answer these questions based on any single December, so let’s look at the past ten Decembers plotted in the following figure. 

Trying to pick a ‘normal’ December is difficult. They generally show decreasing temperatures, although not always. Temperatures actually increased throughout December in 2007. None of them actually match the December climatology. They all show temperature fluctuations, but the fluctuations don’t really show much of a predictable pattern. If we were to make a statement about the weather for this coming December, all we could really do would be to state the climate average (“Decreasing daily average temperatures from 35°F to 25°F…”) plus make a statement about the average temperature fluctuations and their frequency (“...with deviations from that trend around 10°F every 5 – 10 days”). That’s a climate forecast. It’s not a weather forecast.

We’re back to that fundamental difference between weather and climate. The climate average is so abstracted from our actual experience that it’s impossible to feel the climate in any meaningful way. All we feel is weather. It’s only with statistics and averaging that we can experience climate. To get a weather forecast for December, we’ll have to wait until late November when actual weather models can start to make meaningful weather forecasts.

The following four figures should help us get a more intuitive understanding of these differences. They are the averages of the last 2, 5, 10, and 20 years of daily December temperatures. At only two years, you can still see the influence of the warm days from 2013. Early December in 2012 was also warm, and a 2-year average still is subject to the random patterns of weather. We’re not yet looking at climate.

At five years, much of the year-to-year variability is smoothed out.

At 10 years, the temperature plots are even smoother. It’s looking more like the climatology now.

At 20 years, it looks even better. The green line (average daily temperature) lines up pretty closely with the long-term climatology.

But by the time we’ve averaged 20 years together, we’ve moved from the concrete world of daily weather that we can experience into the abstracted world of climate averages. As we increase the amount of time we’re averaging together, we see less of the impact of weather variability and more of the average abstracted climate trend. It’s not until we’ve averaged at least 20 years together (and many climate scientists would rather average 30 or more years together) that we can even talk about climate.

When anyone talks about climate, or a change in the climate, they are by definition not talking about weather. It’s important to recognize that a single weather event may be extremely warm, cold, wet, or dry, but that event cannot be used on its own as a line of evidence in a conversation about climate. To talk about climate you must talk about abstracted averages of weather that cannot be interpreted or perceived as a thing that we can directly experience.

So next time you find yourself in a conversation that confounds weather and climate, remember that the two things are not interchangeable. Instead of responding with confusion to a question like:

Respond with a brief explanation of the differences, perhaps utilize some statistics, and then go outside and enjoy the weather.

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