Weatherdem's Weblog

Bridging climate science, citizens, and policy

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Warming Pause Research Continues

By my count, there are four leading scientific explanations for the short-term slowdown overlaying the long-term rate of global surface warming.  They are:

Enhanced heat uptake by the world’s oceans, especially over the Pacific due to enhanced easterly winds

Sustained small to medium-sized equatorial volcanic activity

Slightly reduced solar irradiance

Natural variability, including the Interdecadal Pacific Oscillation

One interesting aspect to  these explanations’ presence is most researchers believe their explanation is the leading one.  It is a symptom of the state of climate science:  specialization proliferation leads to poor cross-disciplinary communication.  Someone might have this within their purview, but I am currently unaware whether anyone is apportioning relative causality of these explanations together.  Attribution is challenging, of course, but such an effort seems worthwhile to me.

Some recent science updates on these explanations:

Heat Uptake by Several Oceans Drives Pause

Reconciling Warming Trends



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Research: Recent Warming Rate May Be Higher Than Previous Estimates

Posts here slowed down for a bit due to my participation in a poster symposium, a research group presentation, studying for comprehensive exams, and writing thesis chapters.  I will continue to post at a lighter than normal rate until February.

Climate science is a notoriously complex topic.  We don’t measure the climate everywhere with high precision.  Analysis methods have advantages and disadvantages.  Signals take time to manifest, record, and analyze in proper context.  A new paper in the Quarterly Journal of the Royal Meteorological Society, “Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends” by Cowtan and Way illustrate the difference that individual analysis techniques present to researchers.

At issue is the recent perceived slowdown in global surface temperature rise.  Skeptics seized on this situation as their latest new argument that global warming is a hoax.  Science is a slowly evolving endeavor.  Climate change advocates cite decades, centuries, and millenia data to argue that global warming is man-made and accelerating.  Skeptics used ~15 years data to argue the opposite.  Cowtan and Way examined surface temperature data using a method combination, not just one, as groups like Hadley and NASA do.

Here is the study’s result in a nutshell: Applying multiple analysis techniques to a combination of data reduces the bias present in the individual data and techniques.  The authors posit that the Hadley dataset is cool biased because they omit 16% of the Earth’s surface where land-based observations are few to nonexistent.  Here is a comparison of 1979-2012 temperature trends using Hadley’s original data and Cowtan and Way’s combination:

 photo Globaltemptrend1979-2012MetOffice-vs-CowtanWay13_zps2aed74bb.jpg

Figure 1 – Met Office vs. Cowtan & Way (2013) global surface temperature coverage and 1979-2012 trend.

Where is the lack of Hadley data?  It is where the most warming occurred the fastest: across the Arctic.  Even the areas in Africa and South America omitted in the Hadley analysis show warming.  We should all know Antarctica’s condition by now: some areas are warming, some are cooling; overall there is little trend one way or the other over this time period.  But Cowtan and Way include all areas.

How did Cowtan and Way achieve their result?  To head off an obvious skeptic’s argument: they didn’t just make it up.  They employed recognized and widely used techniques.  They employed the first, “kriging“, over land and ocean areas.  This method fills in data points between observing stations and includes a confidence interval around the interpolated point.  One can assess the bias in the result by removing observing stations, performing the interpolation, then using the removed data as verification points.  I’ve used this method in my own past research.

Cowtan and Way also employed University of Huntsville satellite surface temperature data along with available surface data to fill in gaps in missing areas.  Interestingly, Cowtan and Way’s analysis demonstrates that the kriging method worked best over the ocean and the hybrid method worked best over land.  The bottom of Figure 1 and Figure 2 show the result spatially and temporally, respectively.

 photo Globaltemptrend1979-2012MetOffice-vs-CowtanWay13-2_zpsaf60604b.jpg

Figure 2 – Time series of temperature anomalies in the HadCRUT4 data (black solid line), null reconstruction (red dashed line), Kriging reconstruction (green dashed line), and hybrid reconstruction (blue dashed-dotted line).

The bottom panel of Figure 2 shows the 60 month moving average of HadCRUT4 and reconstructed data.  The hybrid result (blue dashed-dotted line) shows a lower anomaly decrease in the past 10 years than the original HadCRUT4 data, which is biased cool due to lack of data points in warming portions of the globe.  There is almost no discernible difference between the Kriging and hybrid reconstruction time series data in the past 10 years.  The 60 month moving average removes high-frequency effects such as the annual cycle and higher frequency ENSO.  The result shows lower frequency signals, such as global warming.

So what processes occurred in the past 10 years to shift the temperature anomaly trend from sharply positive to less positive?  I’ve covered all of them: increased heat uptake by the ocean, especially the deep ocean: Research: Ocean Heat Content Continued to Rise Through 2010, On the global surface warming “pause”, NASA & NOAA: August 2013 4th Warmest Globally On Record; increased small to moderate volcanic activity and reduced solar maximum activity: Research: Volcanic Aerosols Largely Responsible for Recent Warming Slowdown.  Lastly, the Cowtan and Way paper demonstrates that data coverage and interpolation techniques impact results and conclusions.

Compared to HadCRUT4’s 0.046°C (±0.063) decade-¹ 1979-2012 trend, NASA’s 0.080°C (±0.067) trend, NOAA’s 0.043°C (±0.062) trend, and NCEP-NCAR’s 0.178°C (±0.107) trend, Cowtan and Way’s analysis resulted in a 0.119°C (±0.076) trend.  This means the perceived recent warming slowdown could be less than previously thought (higher trend with smaller bias).  Note that all datasets and analysis techniques indicate 1979-2012 warming.  But if you care about how much warming, this paper provides a compelling argument that the significant entities engaged in up-to-date analysis should take a hard look at their internal methodology and update them as needed.

This paper’s results also mean that a need for robust climate policies at every level of government still exists.  Relatively minor methodology adjustments don’t preclude policy development and implementation.  Policymakers may not want to hear that given their propensity to hand off decision-making to outside experts.  The scientific message has not and will not change.


Research: Volcanic Aerosols Largely Responsible for Recent Warming Slowdown

Climate change skeptics used the recent slowdown in observed surface warming to claim that 20th century warming was temporary and that the Earth would return to lower average annual temperatures.  They offered up many potential explanations for the slowdown, none of which make physical sense.  The Sun’s 11-year cycle (often used to explain away warming), a primary argument brought forth, is not the reason: this cycle’s solar maximum is near at hand, yet warming has slowed down recently.

Recently accepted research points to a viable physical explanation.  In addition to oceanic transport of heat to the deep ocean and recent La Nina events, sulfuric emissions from small and mid-sized volcanoes entered the lower stratosphere and reflected more incoming solar radiation than normal.  This research separated the effect of natural sulfur emissions from anthropogenic emissions, using a model, to determine the former had a much larger influence than thought.  Aerosol optical depth (AOD) is a calculated metric used to represent how opaque or transparent the atmosphere is to different radiation wavelengths.  The layer between 20 and 30 km increased 4-10% per year since 2000, which is a significant change from normal conditions – significant enough to have effects on Earth’s climate.

Here is one of the paper’s graphical results:

 photo AerosolOpticalDepth525nm-Neelyetal2013_zps8ba54484.png

Figure 1. Observed and modeled time series of stratospheric AOD from three latitude bands.  Satellite observations are represented by the black line.  Base-line model runs are in green. Model runs with the increase in anthropogenic emissions from China and India are in blue. The dashed blue line depicts a model run with 10x the actual increase in anthropogenic emissions. The model run with volcanic emissions is in red. The black diamonds and initials along the bottom of the plot represent the volcanic eruptions that were included in the model run. (Source: Neely paper; subs. req’d.)

As the caption says, satellite measurements are denoted by the thick black curve.  Note the large increase in AOD (higher opacity) over the tropics in the mid-2000s (b) and the large AOD increase over the northern mid-latitudes in the late-2000s (a).  While not a perfect fit to the observations, the model run with volcanic eruptions (red curve) does the best job of explaining the origin of the SO2.  Individual eruptions are indicated by black diamonds on the bottom of each sub-plot.  The effects of volcanic eruptions on climate are, in a general sense, well-known.  Injections of SO2 into the stratosphere reflects sunlight, which reduces the amount of energy entering the Earth’s climate system.  The difference between one large-scale eruption (e.g. Pinatubo in 1991) or many mid-sized eruptions in a short time-period (see above) is not large as far as the climate is concerned.

This could be good news as far as the climate is concerned, at least in the shorth-term.  If incoming energy were reflected back into space instead of being stored in the system, we can physically explain the observed temperature trend slowdown (see Figure 2) and treat the slowdown as real instead of waiting for that energy to transfer from the oceans to the atmosphere, for example.

There is also bad news however.  From the study (emphasis mine):

The significant portion of the radiative forcing due to increases in stratospheric aerosol from 2000 to 2010, interpreted as a mechanism of global cooling [Solomon et al., 2011], may now be completely attributed to volcanic sources and should not be considered a trend. Rather, the stratospheric aerosol layer should be treated as a natural source of radiative forcing that is continuously perturbed by volcanic injections of a range of sizes, and potentially other sources such as large fires.

 photo NASA-Temp_Analysis_20130131_zpsdfcedaac.gif

Figure 2. Global mean surface temperature anomaly maps and 12-month running mean time series through January 2013 from NASA.

We can see from the 12-month running mean time series (lower-right in Figure 2) that NASA’s temperature index increased more slowly during the latter part of the 2000s than the 1990s.  Neely et al. suggest that there is no physical reason to conclude that slowdown is a trend of opposite sign than that seen throughout the 20th century.  In other words, once the SO2 precipitates from the stratosphere, as it eventually will, the background warming trend will re-establish itself.  Indeed, future warming will likely be stronger than past warming because CO2 concentrations have not decreased in the past ten years.  To the contrary, they have increased at a faster rate than before.  Greenhouse gases have simply had less incoming radiation to absorb than they did 10 years ago due to the recent presence of SO2 in the stratosphere.

Neely’s coauthor Brian Toon had this to say:

Toon of CU-Boulder’s Department of Atmospheric and Oceanic Sciences. “But overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up.”

This situation provides a good example of another aspect of climate policy.  I wrote about geoengineering earlier this year as part of a Polar Sea Ice post (much more discussion took place here).  One proposed mechanism to reduce the impacts of climate change is human injection of SO2 into the stratosphere, which would mimic natural volcanic effects.  If we implemented such a strategy without simultaneously reducing atmospheric greenhouse gas concentrations, then abruptly stopped the injection (due to lack of funds or international controversy), the resulting warming signal would be higher post-injection than pre-injection.  The result would be unprecedented due to the large warming signal such a halt would introduce to the climate system.

In one more respect then, policymakers have wasted the past decade.  Instead of developing and implementing climate mitigation policies, international inaction continued.  Once the atmosphere removes the SO2, the climate signal will be stronger than before.  We cannot and should not rely on future volcanic SO2 emissions to mitigate our GHG emissions.  The lack of robust policies is a choice, but it is not a wise long-term choice.