RealClimate: How likely is the observed recent warmth?

By Michael Mann
25 January 2016 (RealClimate) – With the official numbers now in 2015 is, by a substantial margin, the new record-holder, the warmest year in recorded history for both the globe and the Northern Hemisphere. The title was sadly short-lived for previous record-holder 2014. And 2016 could be yet warmer if the current global warmth persists through the year. One might well wonder: just how likely is it that we would be seeing these sort of streaks of record-breaking temperatures if not for human-caused warming of the planet? Precisely that question was posed by several media organizations a year ago, in the wake of the then-record 2014 temperatures. Various press accounts reported odds anywhere from 1-in-27 million to 1-in-650 million that the observed run of global temperature records (9 of the 10 warmest years and 13 of the 15 warmest years each having had occurred since 2000) might have resulted from chance alone, i.e. without any assistance from human-caused global warming. My colleagues and I suspected the odds quoted were way too slim. The problem is that each year was treated as though it were statistically independent of neighboring years (i.e. that each year is uncorrelated with the year before it or after it), but that’s just not true. Temperatures don’t vary erratically from one year to the next. Natural variations in temperature wax and wane over a period of several years. For example, we’ve had a couple very warm years in a row now due in part to El Niño-ish conditions that have persisted since late 2013 and it is likely that the current El Niño event will boost 2016 temperatures as well. That is an example of a natural variation that is internally-generated. There are also natural variations in temperature that are externally-caused or ‘forced’, e.g. the multi-year cooling impact of large, explosive volcanic eruptions like the 1991 Mt. Pinatubo eruption, or the small-but-measurable changes in solar output that occur on timescales of a decade or longer. Each of these natural sources of temperature variation lead to correlations in temperature from one year to the next that would be present even in the absence of global warming. These correlations must be taken into account to get reliable answers to the questions being posed. [more]