June 6, 2014

Global Warming Decoded

“How do the scientists know climate change is happening?”

I have heard the question posed a number of different times from a number of different people: the weather changes all the time, why do more than 97% of scientists think the climate is changing? The National Climate Assessment Report, released recently by the White House, provides one of the most understandable answers to this question I have seen so I’m going to call attention to it here.

As the report says, “Evidence for climate change abounds, from the top of the atmosphere to the depths of the oceans.”  What is that evidence exactly? The report includes a handy infographic (here’s more on why we like these) that summarizes just 10 of the trends scientists see from their meticulous collection of data “using satellites and networks of weather balloons, thermometers, buoys, and other observing systems.”

Ten indicators of warming world - NCA

“How much has temperature changed?” The NCA reports that globally the temperature has increased by 1.5 degrees Fahrenheit since 1880. To see how clear the trend is, see the graph copied from the report below, which shows the average change by decade, compared to the average temperature for the entirety of the 1900s. According to the Assessment, here in the U.S., it is getting warmer too: U.S. average temperature has increased by between 1.3°F to 1.9°F since 1895.

Temperature change by decade NCA

“Even if it is changing, is the change caused by people?”

So the next question I get is, are scientists sure climate change is caused by people? And again, the report couldn’t be more definitive: “Multiple lines of independent evidence confirm that hu­man activities are the primary cause of the global warm­ing of the past 50 years.” These include:

  • “The burning of coal, oil, and gas, and clearing of forests have increased the concentration of carbon dioxide in the atmosphere by more than 40% since the Industrial Revolution.”
  • “Data show that natural factors like the sun and volcanoes cannot have caused the warming observed over the past 50 years.”
  • “The pattern of temperature change through the layers of the atmosphere, with warming near the sur­face and cooling higher up in the stratosphere, further confirms that it is the buildup of heat-trapping gases (also known as “greenhouse gases”) that has caused most of the Earth’s warming over the past half century.”

The report goes on to describe many of climate change’s impacts by region of the U.S., some of which are occurring today. These are worth looking up on the report’s visitor-friendly website.

Congratulations to John Holdren, the White House’s Science Advisor (and one of my professors at the Harvard Kennedy School), and the team of over 300 experts involved in producing a report on climate change that everyone can understand!