NASA’s James Hansen and a few of his colleagues released their assessment of 2011 global temperatures recently. In short, 2011 was the 9th warmest year in the GISS dataset.
Just as importantly, this situation occurred in the midst of a continuing La Niña event that is of moderate strength. La Niña is characterized by a general cooling of the tropical Pacific waters near the surface; it is frequently referred to as being the opposite of El Niño. As La Niñas progress, global temperatures tend to cool from their normal state. This of course has implications as scientists work to differentiate the effects of natural climate processes and those brought about by humans. If one year’s temperatures are cooler than the preceding year’s (or are warmer), does that mean that global warming has stopped (as skeptics like to say) or does that mean that there are competing forcings that affect the temperatures recorded?
It is the assessment of an overwhelming majority of climate scientists that global warming has not stopped. Instead, the 2nd half of 2010 and all of 2011 were dominated by La Niña events. What does this mean? It means that if the La Niña events had not occurred (and if there were no El Niños either), in other words purely “normal” conditions, 2011 likely would have been warmer than was recorded. This should become obvious in the next 6 months to 3 years as this La Niña dissipates and conditions across the globe respond accordingly. It takes ~6 months for downstream effects to show up in observations after ENSO phases start and after they go away.
Here is Hansen et al.‘s updated figure showing global land-ocean temperatures using an index:
Figure 1. Global surface air temperature anomalies relative to 1951-1980 base period for annual and 5-year running means. Green vertical bars are 2σ error estimates (Hansen et al., 2010). (Source for all graphs: Hansen)
The last black square on the right hand side of the graph is 2011′s temperature index value: +0.51°C. You can clearly see where the 9th highest ranking comes from when viewing this graph. You can further see that 2011 was warmer than 2001, 2004 and 2008 (simply comparing the past 10 years of values), as well as every year prior to 2000 save 1998, the year when the last century’s strongest El Niño occurred.
But I wrote above that large changes can occur year-to-year and this is evidenced by the jagged look to the yearly data in the graph above. So what happens if the data is analyzed in such a way as to remove the yearly signal? Furthermore, can the ENSO and solar cycle signals be quieted down to get a better idea of what the global temperatures are likely doing? Yes they can, as the following graph demonstrates:
Figure 2. Global surface air temperature anomalies relative to 1951-1980 base period for (a) the 12-month running mean, and (b) the 60-month and 132-month running means.
The right panel of Figure 2 demonstrates the results of the removal of the ENSO signal (red line, 60-month running mean) and the solar cycle signal (blue line, 132-month running mean). The addition of more months into the running mean helps to remove more and more noise (to a limited degree, of course). What is left behind is increasingly the global warming signal in global temperature data. A key takeaway is this: the same general result can be seen regardless of the specific temperature dataset employed.
To expand on this topic a little more, here is a graph comparing mean temperature anomalies and the Nino 3.4 index (and index used to characterize the ENSO signal as El Niño or La Niña):
Figure 3. Global monthly and 12-month running mean surface temperature anomalies relative to 1951-1980 base period, and 12-month running mean of the Nino 3.4 index.
Paired with the Nino 3.4 index data, it is very easy to pick out the ENSO influence on the temperature data. Peaks in global temperature anomalies tend to occur during El Niños while troughs in anomalies tend to occur during La Niñas. As you can see, claims that global warming has “stopped” in the past couple of years are not likely to be correct since a prolonged La Niña has occurred during that time frame. One good indicator of whether or not global warming has stopped will be what the global temperature anomaly is ~6 months after the next El Niño peak occurs (likely sometime in the next 3 years).
Another good indicator of whether global warming has stopped or not will be what global temperature anomalies register as the upcoming solar maximum descends from its next peak. As the following graph illustrates, the peak is likely to occur 3+ years from now:
Figure 4. Solar irradiance from composite satellite-based time series. Data sources: For 1976/01/05 to 2011/02/02 Physikalisch Meteorologisches Observatorium Davos, World Radiation Center and for 2011/02/03 to 2012/01/11 University of Colorado Solar Radiation & Climate Experiment. Data are concatenated using the 2010/02/03 to 2011/02/02 period.
It is important to note that the global temperature response to the solar cycle is delayed by ~18 months. So in 4-5 years from now, we’ll have a much clearer idea of the effects of global warming in the 1st half of the 2010s were. That time period will occur after the next solar cycle maximum and after the next El Niño. It strains credulity to think that global temperatures will be lower after those two milestones than they are today.
My thoughts on this are easily understood: it is more likely that global temperature anomalies will continue to exhibit decadal-scale rises than falls in our future (21st century). As I’ve stated many times before, it is also likelier that projected temperature increases are underestimated, not overestimated. We are more likely to read about additional top-10 warmest year on record in our future. That said, I’d be happy to be wrong about all of this. The changing environment we’re living in demands changes to the way our societies function. I don’t believe those changes will be equally catastrophic to everybody around the globe. But all of us will be affected by this phenomenon in one way or another. How we decide to handle those changes will be the key.