Weather vs. climate, and what a difference a few degrees can make

There seems to be some confusion out there about weather vs. climate.  For example, a Virginia Republican Party video urged citizens to call their Congressmen and tell them how much global warming they got during the big snowstorm a couple of weeks ago. But that doesn’t really make any sense.  In simple terms, weather determines whether you need to take an umbrella with you today; climate determines whether you need to own an umbrella.  Weather determines whether you need your down coat today; climate determines whether you need to own a down coat.  Weather determines whether you turn on your air conditioning unit today; climate determines whether you own an air conditioner.  Weather determines whether the plants in your garden have a good day; climate determines what plants will likely thrive in your local environment. 

Climate is the long-term average of weather.  Weather changes all the time; climates are generally fairly stable, allowing us to make long-term decisions based on the notion that the future climate will be like the past.  One unusual weather event does not mean the climate is changing. But many unusual weather events could mean the climate is changing. And climate change will mean that on average, the weather we will have in the future will be different from what we had in the past.   That could even mean that record-breaking snowfall events happen more and more often in Virginia and Washington, D.C.

There also seems to be some confusion out there about global vs. local.  Global warming means that the average temperature of all the places all over the world is increasing.  It doesn’t mean that any particular place is necessarily warmer, it just means that more places have gotten warmer than have gotten cooler.  Just because the east coast of the United States is having a cold winter doesn’t mean the rest of the world is cold.  Seattle just had its warmest January on record, and it's been raining in Vancouver in the middle of the Winter Olympics.  In fact, on a global average basis, last month was the warmest January ever recorded in the satellite data that have been collected since 1979. The world is, in fact, continuing to warm despite the winter we’re having here in D.C.

A U.S. government report concluded that “Global average temperature has risen by about 1.5 degrees F since 1900. By 2100, it is projected to rise another 2 to 11.5 degrees F.”  Even the low end of that range is a pretty significant amount of warming.  On the map below, compare Chicago, the windy city, to more moderate Pittsburgh.  Chicago has an annual average temperature of 49.1 degrees F, whereas Pittsburgh’s is 50.9, a difference of 1.8 degrees.

Annual average temperatures in selected U.S. cities
Source: National Oceanic and Atmospheric Administration: National Climatic Data Center. Comparative Climatic Data for the United States through 2008. Accessed 23 February 23, 2010. Available at

As another example, compare Washington, D.C. to Boston, MA.  Washington is known for its hot, humid summers and its inability to deploy snowplows; Boston is known for its cold winters and pleasant summers.  Washington’s annual average temperature is 57.5 degrees Fahrenheit.  Boston’s is 51.6.  What a difference 5.9 degrees can make! 

Mark Twain said the coldest winter he ever spent was the summer he spent in San Francisco, and L.A. is famous for its warm and sunny climate.  Yet the cities differ in temperature by only about eight degrees. 

It is unclear how global climate change will impact any particular location in any particular season. But it  is clear that overall, climate change will affect our wardrobes, our houses, and our local plant and animal life.  More important, it will affect our economies, our infrastructure and our environment, all of which are suited to the fairly stable climate conditions of the past and will have to adapt to rapidly changing climatic circumstances – unless we begin to take action now to reduce the greenhouse gas emissions that are heating up the planet.  

Judi Greenwald is Vice President for Innovative Solutions