After last year, which tied 2005 as the warmest year since reliable instrument records began in 1880, this year is proving to be considerably cooler at the global level. In fact, 2011 may rank at the bottom, or just outside of, the top ten list of the warmest years on record, potentially leading to questions of whether global warming is abating.

Let me head off such questions right here by saying that long-term global warming continues. The fact that this year is currently running cooler than last year does not in any way invalidate the fact that the world is warming, and that this warming is very likely due mostly to manmade greenhouse gas emissions.

Shorter term climate variability from natural factors such as La Nina - a recurring climate cycle characterized by cooler than average water conditions in the tropical Pacific Ocean - have helped dampen average global surface temperatures this year.

Short-term climate variability is one reason why depictions of global temperatures over time show many spikes and valleys rather than a monotonic climb in which each year is warmer than the previous one.

This year will mark the 35th year in a row to rank above the 20th century average (September was the 319th straight month with global surface temperatures above average). The last year with temperatures below the 20th century average occurred way back during the Ford administration in 1976, according to the National Oceanic and Atmospheric Administration (NOAA).

According to statistics NOAA released last week, average global surface temperatures in September were the eighth warmest on record. Warmer-than-average conditions occurred throughout Europe, parts of Africa, western Russia, the western and northeastern United States, Canada, and Mexico. Portions of eastern Asia and the central United States experienced cooler than average conditions last month.

Temperatures relative to average in September. Red dots indicate areas warmer than the 1971-2000 average, blue dots indicate cooler than average conditions. (NOAA)

Average global surface temperatures during the January to September period were the 11th warmest on record, at 0.94 degrees F above the 20th century average (with a margin of error of plus or minus 0.18 degrees F). When you only look at land temperatures, thereby excluding the La Nina influence in the Pacific Ocean, the year so far appears warmer, at 1.44 degrees F above the 20th century average, which ranks as the seventh warmest such period on record (with a margin of error of plus or minus 0.36 degrees F).

With La Nina projected to strengthen as it enters its second-straight year, it’s unlikely that 2011 will climb much higher up the top 15 list.

Instead of being remembered as the warmest year on record, this year is likely to be viewed as one of the most extreme years - or as one climate researcher recently called it, “the year of living dangerously.” The United States, for example, has experienced a record number of billion dollar disasters, from Hurricane Irene to deadly Southern tornadoes to the ongoing Texas drought and wildfires. Some of these events may be linked in part to global warming, which raises the risk of some types of extreme weather events, such as heavy rainfall and heat waves.