No, a cold snap in the US does NOT mean that global warming has stopped.

This feels like I'm in the movie Groundhog Day.  If there's a cold snap in the US, there's some science denier citing it as "proof" that there's no global warming.  This argument fails for multiple reasons.


First, the good ol' USA comprises 1.9% of the Earth's total surface area (9,629,091 km2 for all 50 states versus 510,072,000 km2 for the entire planet).  While you wouldn't know it from such sources as Fox News or talk radio, weather patterns in the US hardly make a difference to world climate.  For instance, in 2014, the eastern US was colder than the 1981-2010 average, as shown by the UAH satellite temperature data for the entire year.

Figure 1.  Annual average UAH satellite temperature anomaly for 2014.
Despite temperatures in the US, 2014 ranks as the third hottest year in the satellite record, behind 1998 and 2010 and ahead of 2005, all of which were El Niño years whereas 2014 was ENSO neutral.

Table 1.  Top five hottest years in the UAH satellite record
RankYearTemperature anomaly (ºC)
119980.42
220100.40
320140.28
420050.26
520130.24

How can that be if the US was colder than normal? Simple. The rest of the world (98.1% of the planet's surface) was generally warmer than normal—and far more of the planet was warmer than normal than was colder than normal.

Second, global warming hasn't stopped just because NOAA data shows that continental US temperatures haven't risen significantly over the past x number of years (the usual number thrown around is 20 years).  Why?  Take a look.  Here is NOAA temperature data for both the contiguous US and the entire globe since 1990.

Figure 2.  NOAA data for contiguous US and global temperatures 1990-2014.  Dashed lines represent 95% confidence intervals.

The rate of rise is nearly the same: +0.136ºC per decade for US temperatures versus 0.134ºC per decade for the entire planet.  However, the US temperature rise is not statistically significant (p = 0.308) whereas the global temperature rise is (p = 0.0000182).  The difference is the variance in the data.  The US data is more variable than the global data, leading to a larger standard error (± 0.130 for the US versus ± 0.025 for the globe).

The fact that the US temperature increase is not statistically significant does not mean that there is no increase in temperatures.  The increase is there.  What it means is that the observed increase could be due to chance.  However, that fact about US temperatures has no bearing whatsoever on whether or not global temperatures have increased.

The main conclusion from this is that the US, at 1.9% of the planet's surface, does not represent the entire planet.  Just because there may be a cold spell in the US does not mean that global warming has stopped.  As for the reasons why some parts of the Northern Hemisphere are roasting in record highs whereas others are freezing with record lows, the short answer, I wrote a post in January 2014 on that topic.

Comments

  1. Jim,

    I get the fact that the US land mass is only a very small part of the entire global surface and that even if temps are low in North America, that does not mean they are not warmer over much of the remaining world's surface. But as of March 2nd, 2015, the news has featured continuing frigid temperatures and extremely heavy snowfall in the midwest and the Eastern States, including cities subjected to lake snow effects in New York.

    I also know that increased energy in the climate, has likely resulted in more water vapor in the air, and thus may the culprit behind unusually massive amounts of snowfall, but could you tell us more about why the jet stream has apparently moved so far south, allowing cold arctic air to penetrate deep into the midwest? I understand fully that this many be another weather extreme caused by climate change, but could you discuss the reasons why?

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    Replies
    1. Excellent question. I wrote a post in January 2014 that answered your question (http://environmentalforest.blogspot.com/2014/01/polar-vortex-and-global-warming.html). In brief, the speed of the jet stream is determined by the temperature and pressure differences between tropical areas and polar regions. The larger the differences, the faster the jet stream. Warming in the Arctic has decreased the temperature/pressure gradient and slowed the jet stream. That has allowed large waves to develop in the jet stream. Since the jet stream itself is moving slower, those waves become essentially stationary. Those areas in a trough get cold polar air for extended periods whereas those areas under a peak get warm equatorial air.

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