We have had a cold winter. Does that mean the overall trend for global warming is over?
No. Weather conditions at specific locations, including temperature, are dynamic and vary with regional and hemispheric weather patterns. The average global air temperature, which is the sum of regional temperatures, has been gradually rising over the past 100 years. According to NASA, the ten warmest years on record have occurred since 1997. The Fourth Assessment of the Intergovernmental Panel on Climate Change, or IPCC, reported in 2007 that 11 out of 12 of the years from 1995 to 2006 — the exception being 1996 — are 11 of the 12 warmest years since 1850.
Despite the current global warming trend, it is important to remember that not all parts of the Earth warm equally or at the same rate. The Arctic, for example, is warming faster than other areas, while the equatorial region is warming much more slowly. In fact, the Arctic is warming so fast that some climate modelers project an ice-free Arctic Ocean in summertime — possibly as early as 2040, but very likely by end of the century — if emissions of greenhouse gases continue to grow at current rates.
For periods of a few years, regional weather patterns can dominate local temperatures, masking a general upward trend in global temperatures. For example, suppose that the world’s average temperature rises a fraction of a degree in a given year. The prevailing winds where you live might shift into a pattern that could actually make the winter in your area several degrees colder than normal.
Even the worldwide trend is not a smoothly rising line, but rather a signal that fluctuates, as shown in the graph above. Over a short timescale of a couple years, one can expect temperatures actually to fall below the average value, even though over a longer timescale the average is still rising. There is, as one would expect, natural variability within the overall trend.
An example of the dynamic nature of climate variability was evident in 1998 when the average temperature lurched upward significantly (more than .72ºC [1.3ºF]). It turned out that the Pacific El Niño current was especially strong that year. Afterward, temperatures actually dropped a bit for the next couple of years.
So, while measurements show that the average temperature of the planet has risen over the past century and likely will keep rising over the next 100 years, we should not expect every year to be warmer than the year before at every spot on the Earth. In some places, and even for the Earth as a whole, there may be years when it is actually a little cooler than the previous year. Nevertheless, the data indicates that over the long term, we’re heading toward a hotter world — a reality that is further supported by our knowledge of the relationship between carbon dioxide, or CO2, and other greenhouse gases and temperature, as well as our knowledge of the climate system.