Scientists have typically been cautious when discussing the link between a single extreme weather event and climate change, preferring to focus on broader trends. Previous work, including a paper I wrote with Jay Gulledge four years ago, described a framework for how to think about the link.
But a new report from the National Academies of Sciences (NAS) is making the connection more clear by defining the relative contributions of climate change and other natural sources to the risk of individual weather events. The NAS report – an exhaustive, systematic examination of the peer-reviewed literature – finds high confidence in attribution studies linking individual extreme heat and cold events and climate change, and a more moderate confidence level for several other types of events.
Climate change is making extreme weather more likely. But individual weather events like heat waves or hurricanes are always the product of several risk factors, such as El Nino, climate change or other natural variability, akin to how a poor diet and smoking increase the risk of poor health later in life.
Extreme event attribution attempts to quantify the influence of climate change in comparison to other factors. Determining to what extent climate change strengthened or weakened the event can further our understanding of how much impact climate change is having on our weather.The NAS report assigns a confidence level to the climate impact for a variety of weather events based on three supporting lines of evidence:
- The physical mechanisms that link climate change to a particular extreme
- The length and quality of the observational record showing the baseline risk level and changes to date
- Computational climate modeling showing an increase in risk for a class of extreme event
The report finds the strongest links to climate change for extreme heat and cold, with the highest level of confidence across all three lines of evidence. Drought and extreme rainfall have medium confidence for physical understanding, observation and modeling. Extreme snowfall has medium conference for two out of three, physical understanding and modeling, while the observational record for snowfall is poor.
Image courtesy NOAA
This visualization from NOAA shows much warmer than average or record warm temperatures across much of the globe in 2015, the warmest year on record.
The data are in, and 2015 was officially the warmest year globally ever recorded. We’ve been keeping temperature records since 1880. The last time the record was broken? 2014.
What’s interesting is just how much warmer 2015 was. The observed annual average surface temperature was more than 1.8° F (1° C) above the 19th century average, according to the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA). That’s already half the warming countries have agreed to as the international limit.
And 2015 was about a quarter of a degree Fahrenheit warmer than 2014. That might seem small, but it’s actually huge when compared to the year-to-year differences observed in the record.
A strong El Niño, when the surface ocean in the Eastern Pacific basin warms, contributed to the record warmth of 2015. But even compared to other El Niño years, 2015 set records. The agencies reporting the data attribute this to the long-term warming trend due to the increase of greenhouse gases in the atmosphere.
As with all climate and weather data, the 2015 data shows some variability. Not all locations set high temperature records, and parts of the North Atlantic Ocean actually set a cold temperature record.
In the contiguous United States, 2015 was the second warmest year on record, with 2012 still holding the top spot. It was the 19th consecutive year that the annual average U.S. temperature was more than the 20th century average.
A recent Senate hearing highlighted some of the progress U.S. communities are making, and the major challenges they face, in better coping with costly extreme weather events — including those, such as heat waves and coastal flooding, whose risks are heightened by climate change.
Sen. Tom Carper, chairman of the Homeland Security and Governmental Affairs Committee, noted that the “frequency and intensity of these extreme weather events are costing our country a lot - not just in lives impacted – but in economic costs, as well.” Nearly 130 weather-related events in 2013 caused more than $20 billion in losses in the United States.
Extreme weather is costly, not only to federal, state, and local governments, but also to businesses and individuals.
Much of the Senate testimony echoed key findings in our report, “Weathering the Storm, Building Business Resilience to Climate Change.” Three key points made at the hearing were:
As President Barack Obama prepares to deliver his State of the Union address, we believe it’s a good time to take a look at the state of our climate: the growing impacts of climate change, recent progress in reducing U.S. emissions, and further steps we can take to protect the climate and ourselves.
The consequences of rising emissions are serious. The U.S. average temperature has increased by about 1.5°F since 1895 with 80 percent of this increase occurring since 1980, according to the draft National Climate Assessment. Greenhouse gases could raise temperatures 2° to 4°F in most areas of the United States over the next few decades, bringing significant changes to local climates and ecosystems.
This week’s brief but bitter cold snap over more than half the country prompted intense discussion about the polar vortex ranging from educational to bombastic.
|Figure 1: A depiction of the “average” polar vortex on Jan. 6. The winds of the vortex correspond to the narrow “rainbow” areas. The map is an average of the upper atmosphere’s “topography” (specifically, the 500 millibar height) from all the January 6ths between 1980 and 2010.|
|Figure 2: The polar vortex on Jan. 6, 2014. The ridge (“R”) and trough (“T”) responsible for relatively warm weather in much of the West and bitterly cold weather in the Midwest and East have been labeled.|
So let’s be clear: The cold snap this week was unusual but not entirely unprecedented. A few super-cold days don’t disprove global warming, just like a day of rain doesn’t end a drought. At the same time, we don’t yet know whether climate change will change the odds of future outbreaks of bitter cold. Research is still underway, and as of now, we shouldn’t necessarily expect these events to be more or less frequent in future winters.
Here’s a Q&A to cut through the hype:
- What is the polar vortex? The polar vortex describes the air circulating aloft (thousands of feet above the ground) about the North Pole, and its extent is marked by a ribbon of strong winds that is often called the “jet stream.” (We most commonly focus on the North Pole, but a similar circulation is present around the South Pole, too).
In the map (Figure 1), which is from the point of view of the North Pole, the vortex corresponds to purple and blue colored areas. The band where the colors change from blue/purple to red/yellow indicates the location of the jet stream, or the outer edge of the vortex. Winds are strongest where this color gradient is tightly packed (e.g., over the Pacific Ocean and North Atlantic Ocean). It tends to be quite cold at the surface below the purple areas, and warmer under the red/yellow areas.
It’s important to note that this figure is an average of many winter days. On any given day, we would see a number of deviations from this average pattern.
- What happened this week? Comparing this week (Figure 2) to the average picture (Figure 1), we can see that the purple area of the vortex has contorted and moved farther south. Along with this pattern, there are substantial “wiggles” in the jet stream. These deviations in the circulation helped bring cold air into the continental United States that normally stays in northern Canada and the Arctic. Meteorologists look for these wiggles, called “ridges” and “troughs” (“R” and “T” on the map) when putting together a forecast. While the trough brought notable cold to the Midwest and the East, the ridge has kept parts of the West warmer than average and relatively dry (much to the dismay of skiers).