A year of weather extremes?

Through 2010 and 2011 in particular, weather extremes seemed to dominate the headlines. Extreme drought, rainfall, flood and wind all played a role in making the period one of the most expensive in terms of damage to infrastructure. In some locations there was also significant loss of life. It was also a time that saw the subject of extreme weather events rise up the climate change agenda, with numerous academic papers, blogs, seminars and campaigns focused on the issue.

Certainly as the atmosphere moves from one steady state to (presumably) another and one which is warmer and therefore has more energy, weather volatility should increase, at least during the period of transition. This is true in any control system where there is a change in set point (not exactly what is happening in the world, but analogous). The picture below is fairly typical, with large swings in response as the system adjusts to the change.

So we might well expect to see an increase in extreme weather events and many are now pointing to recent events as evidence. The problem here is that there have always been extreme events and there have also been previous periods of bunched extreme events. This may be driven by climate cycles, such as the El Nino Southern Oscillation (ENSO). A period that shows many similarities to the last two years is 1974-75 when there was a very strong La Nina event, such as the one we are currently experiencing.


In the timelines above the near back-to-back El Nino events of the 1970s and 2010s are shown in blue (also see them in the chart above the timeline in blue) and various extreme events are shown in red. Much similarity exits, although the severe droughts that have been experienced in the southern US states didn’t show up at all in the 1970s. In fact the Texas drought has been shown as exceptional by any standards.

With so much focus on extreme events and a further focus by many on an apparent plateau seen in global temperatures in recent years, are we perhaps missing some clearer signals buried in the data? One such signal, which got very little media coverage, was published by the WMO at the very end of 2011 and shows last year to be the hottest ever, for a La Nina year (which are typically cooler). In fact every La Nina year over the past 40 years has been warmer than the previous one.

Over six decades and taking just the La Nina years (chart above) there has been a temperature movement of 0.7 deg.C, or 0.12 deg.C per decade. This is somewhat less than the climate sensitivity indicated by the IPCC, but equally it may only be indicative of what is probably the bottom edge of the span of temperature change. It is nevertheless an important trend to understand and follow. Extreme weather events also deserve considerable attention, but there needs to be some increased diligence when it comes to immediately associating them all with climate change.