Is climate change slowing down?

Question: I heard climate change has been slowing down. Is it?

Answer: No!

For the last decade or so, there has been a lot of talk about a global warming ‘hiatus’ or slow down, even though the last decade was the hottest one of the instrumental record period (back to the late 1800s).

But was the temperature rising throughout that period such that the end of it was even hotter than the beginning?  Or has the rate of further warming slowed down such that we might have more time than was thought to debate and mull over the pros and cons of taking action now?

no slow down graph

Source: http://www.ncdc.noaa.gov/news/recent-global-surface-warming-hiatus

People arguing against taking action on climate change made much of the so-called ‘hiatus’, claiming that global warming had ceased altogether, was somehow self-limiting, or just wasn’t as much of a problem as the great majority of climate scientists, and, increasingly, a majority of people throughout the world thought it was.  There have been several answers to this, all valid, but a new study just published in Science by National Oceanic and Atmospheric Administration (NOAA) scientists Thomas Karl and colleagues now reveals that the very idea that there was a slow-down was partly a calibration error.

Over 70% of the surface of the globe is ocean, and so sea surface temperatures (SST) are hugely important in calculating the average surface temperature of the globe.  But there are not the kind of fixed weather stations that we have on land to calculate the average.  Instead, measurements have been taken by ships as they traveled the seven seas, and, more recently, by both ships and buoy networks.  Therein lies the source of many calibration challenges, because different ships used different measurement techniques, and ships and buoys in general have tended to have different biases.  In fact ships tend to produce slightly higher temperature readings than buoys (the buoys are thought to be most accurate), and the increasing reliance on buoy networks during the last 15 years had created an artificial flattening of the apparent global warming rate.  When Karl et al account for this difference between ship and buoy collected data, the more accurate calculation of global temperature shows no significant slowdown in warming in the past 15 years compared to the last half of the 20th century.

noaa ocean heat content graph

Source: https://www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/

For most climate scientists, this is no great revelation.  It is important to remember that what is usually reported as global temperature is the surface temperature of the globe.  That’s not an unreasonable reference point given that we humans spend most of our time there, but it is still an incomplete picture.  We already knew, for instance, that temperatures within the upper 0-2000m of the ocean have been rising, especially those in the upper 700 m, and that about as much additional heat was being loaded into them each year as the climate models predicted.

And then there is the fact that while humans are driving strong warming today with our release of greenhouse gasses into the atmosphere, there are also natural cycles of ocean and atmospheric processes that can cause surface heating and cooling for periods of a decade or even longer. Such natural cycles are still happening, superimposed on the rising global temperatures caused by human impacts.  These natural cycles sometimes add and sometimes subtract from the consistently upward global warming effects resulting in observed rates of global warming fluctuating in multi-year periods, with some periods slower and others faster than the mean rates of warming predicted by climate models.  For example, the last decade has actually been dominated by La Nina conditions in the equatorial Pacific Ocean, and these are associated with cooler SST, temporarily dampening the apparent rate of warming through the decade.  2015, however, has moved into a condition of El Nino, when winds and the pattern of mixing with deeper waters result in higher than average SST in the equatorial Pacific.  Many weather forecasters are predicting that, if the El Nino continues, this could make it a real scorcher of a year.

And finally, there is Arctic Amplification, in which the Arctic is observed to be warming at a faster rate than the rest of the world.  But exactly how much faster?  This is challenging because there are so few measurements covering the harsh conditions around the either Pole.  A study last year by Cowtan and Way, however, developed a robust way to calibrate and use satellite data to fill in the great data holes at the poles. With the new data coverage they found the poles to be warming even faster than was thought, and including this raises the observed rate of global warming overall.

So there is no reprieve, there is no hiatus, there is no observation that suggests the climate model predictions for the coming century are in any way exaggerated.  Addressing climate change is an urgent challenge that that must be tackled immediately if we wish to preserve the globe as we know it now.

  • Karl, Thomas R; Arguez, Anthony; Huang, Boyin; Jay, H; Mcmahon, James R; Menne, Matthew J; Thomas, C; Vose, Russell S; Zhang, Huai-min. 2015. Possible artifacts of data biases in the recent global surface warming hiatus. Science, vol 4, p1-7
  • Byron A. Steinman, Michael E. Mann, Sonya K. Miller. 2015. Atlantic and Pacific multidecadal oscillations and Northern Hemisphere temperatures. Science, vol. 347  p2269-2272
  • Cowtan, Kevin; Way, Robert G. 2014. Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends. Quarterly journal of the Royal Meteorological Society. July p1935-1944


Category: Climate Change Q&A

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