Record-breaking temperatures seen in December
New Jersey experienced the warmest December temperatures in recorded history during 2015, according to the National Weather Service.
The warm weather was due to a jet stream, or narrow, fast air current, shifting much farther north than usual, bringing in warm air from the southeastern United States, said Anthony Broccoli, a professor in the School of Environmental and Biological Sciences.
This weather is an example of random weather variability, said Alan Robock, a professor in the Department of Environmental Sciences.
“Weather is the variable, and climate is the statistics of weather,” he said. “Sometimes we’re in a pattern where the winds from the south flow more often than average and it gets warmer. Sometimes it’s colder. February of last year was much colder than average for the same reason."
Such high temperatures should be expected to be normal for this reason. But it does not give insight to the rest of this year’s weather, Broccoli said.
“We have had warm Decembers followed by cold conditions for the rest of the winter, and vice versa,” he said. “The long-range forecast for the remainder of the winter is for warmer than normal temperatures, but we can't really say that the warmth of December is a cause.”
Weather is predicted using complex computer programs, which take observations from around the world and calculate future weather changes based on physics understandings, such as conservation of energy, how clouds are formed and the amount of energy coming in from sunlight, Robock said.
Observations are taken from satellites, surface observations and weather balloons. Due to the chaotic nature of the atmosphere and limited size of computers, it is impossible to make perfect weather conditions beyond two weeks, Robock said.
These limitations can produce errors that can compound, making weather predictions imperfect, he said. Despite this, forecasts are still becoming more reliable than in the past as computers become more powerful and observations get better.
“There’s a small difference between air going up, causing a small cloud and causing a thunderstorm. If you don’t get it exactly right, you might get a thunderstorm where you didn’t expect one, and that puts energy in the atmosphere and gives you a different future than you thought,” he said.
Although the temperatures in December on their own are nothing to worry about, the warm weather throughout 2015 is, Robock said. Variability accounts for deviations from average weather temperatures in a short term, but not over longer periods of time.
Last year was announced as one of the warmest recorded years since 1880, and that long-term climate change is what people should focus on instead of comparing the temperature in a given month, he said.
Over the last 135 years, the globe has been warmed by almost 0.02 degrees Fahrenheit per year. The globe is warming faster now, so another two degree increase in global temperature will not take 135 years, he said.
“(Last year) was particularly warm because of the El Niño in the Pacific, so a lot of energy that was stored in the ocean was put up in the atmosphere,” he said. “Sometimes that happens, and sometimes there’s La Niña, in which a lot of energy goes into the ocean.”
There is weather variability due to the random changes in weather and ocean circulation. In addition, there is the slow gradual warming of the globe due to greenhouse gases being put in the atmosphere, he said.
Hurricane Sandy, for example, was more dangerous because of the rise in sea level and the extra energy from the warmed ocean, he said.
“The really warm weather in December is not global warming,” Robock said. “It was really cold yesterday, but that doesn’t mean global warming doesn’t exist, because those are natural weather fluctuations. But on top of that, the globe is warming with lots of negative impacts.”
Editor's Note: A previous version of this article said that February 2015 was warmer instead of colder than average. It also said climate change caused the Earth to warm "1 percent of 1 degree" every year instead of 0.02 degrees Fahrenheit per year.