Claim: Global warming has stopped since 1998 (or 1999, or 1995; I've seen them all).

Why this claim is wrong: This claim is wrong for a number of separate reasons which, combined, indicate the rate of warming is actually increasing - as indeed would be expected given the relentless rise in CO2 levels. So let's begin.

The notorious Richard Lindzen is the originator of this one, thereafter widely repeated on the denialist blogs for years now. Cherry picking is the first offense - a pretty damning indictment detailed in this article. We showed in Astro 7 class the danger of "lying with statistics". This post shows that, applied to this kind of

temperature data. The "since 1997" claim is extreme cherry-picking. Choosing 2009 back to 1997 is the longest period you can get away with and still just barely fail the standard criterion (95% confidence interval) of conventionally accepted statistical significance, due to the shortness of the interval. It also neatly begins with the hottest El Nino on record (1997/8) and ends on "La Nina" years (~2008). "P-hacking", they call this method of lying with statistics. La Nina years are cooler than average, as the equatorial Pacific surface waters go through the opposite end of the ENSO oscillation. The disingenuousness of it all is discussed here. Note too that in the U.S., 2010 was the hottest year since climate records began, and that 2014 was globally the hottest year on record. This despite the fact that the sun, over most of the '00's, has been going through the longest and deepest solar minimum (and therefore lowest solar luminosity) in a century, and the weakest solar maximum in a century as well. When volcanic, solar cycle, and ENSO oscillations are removed for the past half century, the underlying anthropogenic warming trend is as strong now as ever, as this 2 minute video shows. These facts are ignored by the denialists like Fred Singer, who continue to use it for any time period that suits their agenda . At right is the trend since 1880, not long after the first oil well was drilled in the U.S., and decide for yourself whether global warming has really stopped. Yet there are numerous denialist blog sites that still

claim we're headed into another Ice Age. Even assuming the global average surface temperature were an accurate measure of Earth heating, the so-called "pause" fails any statistical test of significance (see graph at left). I've heard a local denialist make the facile claim in front of his students "you need 50 years to say anything", as a way to dismiss all evidence for human-caused global warming. And yet 17 years is now supposed to be enough to show human-caused warming is a fraud? Really?

Next, Foster and Rahmstorff (2011) show that when known non-greenhouse influences (solar luminosity changes due to the unusually low solar minimum of 2008, the El Nino/Southern Oscillation in the Pacific Ocean, and volcanic aerosols) are included in the estimate of global warming, that there is NO change in the rate of warming of global surface temperatures. See graph at right. What will happen when the ENSO cycle goes back to the El Nino phase? We can expect a sharp rise in the global average temperature.

Another nice demonstration that there has been no drop in the rate of temperature rise, is to plot temperatures for La Nina years, for El Nino years, and for "neutral" (neither El Nino nor La Nina) years with different symbols, then look for any trends when this normal Pacific Ocean variation is removed. The result (Nielsen-Gammon 2012) shows the trend in each of these three categories each have the same upward slope (graph at left).

(a) Global average ocean temperature vs. depth in meters. Note how hotter surface waters associated with cooler deeper waters and vice versa. (b) Sea surface temperatures and 160m deep temperatures. Note how they are inversely correlated as the Pacific goes through El Nino/La Nina cycles. From Roemmich & Gilson (2011)

Update 2013: This bogus claim continues in the right-wing and other climate denialist blog world. Published research in the past two years adds clarity on why the atmospheric temperature rises do not happen at a perfectly constant rate. Here's the streamlined explanation - greenhouse heating is to the atmosphere, and the atmosphere transmits this heat to the ocean surface. The surface ocean then conducts and convects heat to the deeper ocean. The transport of surface ocean heat to deeper layers does not happen at a steady constant rate, and the characteristic time scales for the variations are relatively long, since oceans are large, especially the Pacific.

This is the Pacific Decadal Oscillation (PDO). Heat transport to the deep ocean is easier when the thermohaline gradient is weaker, and relevant physical and thermal scales mean this is variable on time scales of a decade or so on average (there is an interdecadal variation of longer duration as well, both are visible in the data at right). When the gradient is less, surface waters are cooler, and heat transport from the atmosphere to the ocean is easier, which lessens the temperature rise remaining in the atmosphere. In the other half of the cycle warmer lower density waters sit on the surface, inhibiting vertical transport of heat, and the atmosphere retains its heat more easily and air temperatures rise more rapidly. The PDO was in the 'warming' cycle (when El Nino events predominate) in the late '80's to '90's, and transitioned to the 'cooling' part of the cycle (when La Nina's are more common) after the large 1998 El Nino and continues today in this state. If there were no dominant greenhouse warming, one would've expected global climate to cool. It has not. This recent 'cooling' phase of the PDO has only succeeded in reducing the rate of global surface temperature rise, temporarily. The general phenomenon of ocean heat transfer changes with the ENSO and PDO wind patterns is well illustrated in this 2-min video by climatologist Matt England.

2012 was by far the hottest year on record in the U.S., and 10th hottest globally. We can expect when the PDO switches back to the 'heating' part of the cycle, global temperatures will rise more rapidly once again. (longer explanation with links). The cherry-picking aspect is well shown through 2012 in this lecture (skip forward to the segment from time 9:00 to 11:40, but the entire lecture is well worth your listen.).

When these other climate-relevant effects are included, we see no change in the rate of Earth heating. However, we may be getting a bit of a break from the sun, which entered a deep minimum, which carries with it reduced solar luminosity (albeit only ~0.1% or so), and we can hope this will continue.

Update Dec '13
There's a new analysis in 2013 which finds that even without the deep ocean heat transfer and solar minimum influences, global warming does not appear to have slowed during these past ~15 years. The global surface temperature data "cherry picked" to try and make the "global warming has stopped" claim is the HadCRUT4 dataset, which is only available for about 84% of the Earth's surface. The missing areas most importantly includes the Arctic, which satellite data shows is most certainly warming more rapidly than the rest of the globe, most obviously by the dramatic loss of ice. Reconstructing temperatures in the missing regions in two independent ways using satellite data, show that global temperatures have been rising 2.5 times faster than the more patchy earlier data indicated (Cowtan and Way 2013). A 3 minute video of this data issue is here.

Also this year, Neely et al. 2013 find additional evidence that sulfate aerosols from even small volcanoes, which cause cooling of the surface, are important in explaining the slowing of warming since 1998.

You can read more on debunking this "Great Pause" (sic) here.

There's yet another, entirely independent way to debunk the "global warming has paused" nonsense -the trend in sea level. Sea level rises for only two reasons; continental ice melt, and rising ocean temperature causing thermal expansion. Both are governed by elementary freshman physics well understood for centuries. There are very short term oscillations in local tidal gauges due to geoid changes, storms transferring water from ocean to land temporarily, and tides, but the trend is clear - there has been NO pause in sea level rise and therefore by implication, no pause in the rate of absorption of heat trapped by the greenhouse effect on Earth.

Update 2014
A new paper by Bunde et al. (2014) in the Publications of the National Academy of Science finds an advance indicator of the arrival of El Nino is contained in particular tropical Pacific Ocean vs. air temperatures, and has had a 75% success rate. The upturn in global temperatures predicted by the arrival of El Nino may happen as early as late 2014, although the strength of the El Nino isn't well predicted as I write this in mid '14.

Also, this paper in Nature: Climate Change (Riseby et al. 2014) shows that climate model runs which have past natural variation in the ENSO cycle in phase with actual observations, confirm that there is no pause in global warming. I emphasize for clarity here - that the ENSO cycle has a significant "chaotic" element and we cannot predict (yet) more than a few months in advance how the ENSO cycles of the future will arrive. So when climate modeling is done, we begin with initial conditions and evolve the models forward and we observe the ENSO-like cycles evolving, but its detailed arrival times cannot be taken at face value for a random climate model run. Therefore, to properly compare with past data, one can take an ensemble of those climate model runs with slightly different but realistic initial conditions and identify those whose subsequent ENSO cycles are most in agreement with actual ENSO cycle arrival times, and then compare the global temperatures from that subset of models. Choosing those model runs which have their ENSO in phase with the real ENSO then allows the other climate factors to show their contribution more accurately - including the human input. This is entirely legitimate and there is no claim that we can predict the ENSO cycles detailed arrival times. In the same way, one can use the laws of fluid mechanics and see all the phenomena of a rushing mountain stream - vorticity transport, eddy formation and eddy shedding from rocks.... but one cannot predict the timing of the exact formation of individual actual eddies on into the future. That's like "weather" (which has an element of chaos) vs "climate" (which has much less so).

Update Dec '14
A new study (Ridley et al. 2014 in GRL and discussed here) includes better data on volcanic aerosols over past decades. Prior to this, climate scientists had only considered the effects of major volcanic eruptions. There have been none since Mt. Pinatubo in 1992. But the more numerous minor eruptions can still add aerosols to the lower stratosphere. Combining data from satellite and ground sources, shows that more of these aerosols have been added in the past 1-2 decades than previously, and account for 0.05 - 0.12 C of additional negative climate forcing compared to prior decades. This is most of the observed 0.13C decrease in the still-positive rate of global surface warming since 1998. There is no reason to suppose that excess minor volcanic eruptions should continue to help cool us going into the future. This is a significant addition to the climate budget and should get more attention in future evaluations. Together with the other reasons for the apparent slowing of warming in the past 17 years, correcting for these non-anthropogenic and sparse-data causes indicates that in fact the rate of human-caused global warming is accelerating, not declining.

Finally, Note that denialists like Anthony Watts simultaneously bash climate scientists for having inadequate simplistic climate models which therefore can't be trusted, and then (here) bash them for more careful detailed studies which they then dismiss as post hoc explanations for why temperatures did not rise. This sort of convenient flip/flop is perhaps the most telling evidence that such people are not evidence- and truth-oriented, but instead choose whatever "argument" will advance their agenda.

Update Feb '15
Climatologist Kevin Cowtan has a new video showing how raw temperature data is adjusted to remove errors, and how you can verify for yourself with the actual raw data that the scientists have done a good job, and that recent tabloid claims of "fraud!" are outright lies.

NOAA describes new study published in June, showing that indeed there has been no slowdown in global warming. Subtle biases in measurements of sea surface temperatures (including bouys vs. ship-

based) and land-based temperature coverage (e.g. missing Arctic temperature data very time dependent), indicate there has been no change in warming, even without considering the cooling effects of PDO and solar activity - see graph at right (Karl et al. 2015) .

The climate deniers who want you to conclude "climate change has stopped, nothing to worry about. Don't listen to those 'alarmists'" want to divert your attention from what has happened on Earth during this so-called Great Pause: Greenland ice loss sharply accelerated, more than doubling during this period (graph in billions of tons per year). Arctic Ocean ice cover loss has sharply accelerated, both in volume, (and here in 2015) and in area coverage . The large Ilulisat glacier in Greenland has lost more ice in the 2001-2013 period than in the previous 100 years (see 10 min into this video). And the same for Antarctica, for glaciers worldwide, and worse if, and when, the West Antarctic Ice Sheet loses its grounding line (which it now has, discovered in 2014) And of course, much more....

Update 2015

The 16 year cooling effect of the negative phase of the PDO has ended as of 2014, and in 2015 we have transitioned sharply into the warming phase of the PDO. Just as expected, global temperatures are also rising sharply. 2014 was the hottest on record, and 2015 through the first half, is on pace to smash that record, and is fully 0.36F hotter Jan-June than any previous year.

Indeed, 2015 was by far the hottest year on record, as the Berkeley Earth group's plot (they've been the first to officially plot the 2015 data) shows. This new post details the fudging of satellite data done by the climate denialists to try and minimize the trend in surface temperatures from satellite data. Note that as of now, Earth global average surface temperature is fully +1.2C above approximately pre-industrial temperatures (that being a flat line fit to first quarter of the graph at left, before fossil fuel burning became significant).

Update 2016

Despite the fact that 2014 and then again 2015 are setting new records by a wide margin for the hottest years on record, this climate denial claim keeps getting thrust into the media. The latest example is Republican senator Ted Cruz intimidating professor David Titley, Cruz being the chairman of the Senate Committee on Space, Science and Competitiveness (yes, that's really the name of the committee). Note the difference between the chart held up by Cruz, and that held up by Titley, which Cruz tries to rush past once it is pulled up suprisingly by an aide to Titley. Great example of how our politicians work. Where did Cruz's faulty temperature record come from? J. Abraham's piece in the Guardian summarizes the work of Po-Chedly et al. 2016 on how University of Alabama's Roy Spencer and John Christy fail to account for orbital decay in satellite temperature measurements. This affects the assumed-orbit calculated time of day the satellite is measuring temperature. Since daily temperature swings are significantly larger than the signal in average global temperature. Failing to account for this gives a spuriously lowerer and lower temperature reading because the reading happens later and later in the time of day on the ground vs. that assumed from the satellite time-stamp. It's a large effect; a factor of 4 in trend . An equally serious problem with interpretting the satellite data as surface temperatures is that - it is not. It is the temperature 20,000+ ft up in the troposphere. Since the strongest temperature rise due to greenhouse warming will be where the optical depth of greenhouse gases is strongest, at the very bottom of the atmosphere were we, the oceans, and the ice all live, and since greenhouse gases cause a COOLING of the stratosphere above the troposphere, it is not surprising at all that the mid-troposphere temperatures are not rising as fast as is the Earth's surface. The chief scientist RSS who designed and operate the satellite, describes the distortions Ted Cruz is guilty of, here. It is very interesting that such incredible errors persist from people with a strong political agenda and funding (see links to these Roy Spencer and John Christy above) motivating an obvious and consistent denial of climate change. This erroneous temperature record is invariably presented as "fact" by climate denialists in the Republican congress and elsewhere to "prove" global warming has stopped. There are other significant corrections needed for satellite measurements to be able to infer surface temperatures, since the actual data is from oxygen emissions at certain wavelengths along the entire path length from satellite through upper atmosphere to the ground. All of this was ignored by Cruz's committee. Climate scientists have responded to this disgraceful political theater here.

Nature has responded as well, as a new study just published by (Gleckler et al. 2016) shows that the amount of heat absorbed by the oceans, (where 93% of all greenhouse gas heating has ended up), has doubled just since 1997. Put another way, the entire period from the beginning of the fossil fuel era in 1865 till 1997 the ocean absorbed 150 zetajoules of energy (that's equivalent to over 2.3 billion Hiroshima A-bomb's worth of energy), and absorbed that same amount of energy from just 1997 to 2015.

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