This article is a little bit of a tangent to my regular articles. I am presently doing an online course - Climate Change: Challenges and solutions - offered by the University of Exeter (UK). So please indulge me as I also use this blog for some climate course work. This article is for week 3, section 4.4 of the course on 'IPCC Fifth Assessment Report'.
The IPCC recently published their Fifth Assessment Report (2013). Does the recent change in global temperatures reflect the predictions of climate models? Search for other feedback in the media and focus on the role of climate models and then try to address the discussion question: Does the recent change in global temperatures reflect the predictions of climate models?
An interesting question, because recent global mean surface temperatures over the last 15 years from 1998 show only a very slight warming trend. In the HadCRUT4 temperature data series the trend is 0.04°C per decade over 1998–2012, compared to 0.11°C per decade over 1951–2012. This is used by climate sceptics to argue that 'global warming has stopped'.
So have climate models predicted this slow down in temperatures?
Not per se. But global climate models do include natural variability cycles as part of their long term average trend. In some periods of a similar length of time warming is greater than the average, and in some it is less or even shows a measure of cooling. This can be seen even in the last 100 years. So, yes, this recent slow down in global temperatures over the last 15 years forms part of long term climate model trends.
First we need to understand that global mean surface temperatures is just one measure of the global warming process. Sea levels in this period have continued to rise, glaciers and ice sheets have continued to melt, oceans have continued to warm. Solar variability also reached a low in 2012, so slightly less energy was reaching the earth. Several smaller Volcanos added a net cooling effect accounting for up to 25 per cent of this hiatus. There is also substantial internal variability in the climate system in decadal length cycles such as the Pacific Decadal Oscillation, El Nino Southern Oscillation (ENSO),in the fluctuation of the Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO).
It is also significant that this last decade has been the hottest decade on record. Dana Nuccitelli argues that Global warming is unpaused and stuck on fast forward, according to the latest research (see below).
One of the clearest scientific explanations has come from Professor Andy Pittman from Australian National University where he asks Is global warming in a hiatus? Are our politicians being lulled into a false sense of security?
Figure 2: The GISS data, with El Niño and La Niña conditions highlighted. Neutral years like 2013 are gray. Source: NASA via Climate Crocks.
IPCC and evaluation of climate models
So what does the IPCC say? The latest IPCC AR5 review was published on 27 September 2013 and in the IPCC AR5 Summary for Policymakers talks about the evaluation of climate models.
As scientists obtain more data and insight into processes the models are progressively getting more complex but also much better at replicating observed data and making future projections. "Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions (very high confidence)." it says. It continues:
The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observedtrends over periods as short as 10 to 15 years (e.g., 1998 to 2012). {9.4, Box 9.2}
The observed reduction in surface warming trend over the period 1998 to 2012 as compared to the period 1951 to 2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from natural internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that natural internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of natural internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effects of aerosols).
Dr Scott Power, principal research scientist of the Australian Bureau of Meteorology and IPCC AR5 Coordinating Lead Author of Chapter 11, discusses the role of climate models and the IPCC AR5 report, identifying the issues involved:
Let's update the IPCC analysis which lags scientific research by up to 2 years due to time limits with preparing the review and the consensus process.
Internal variability and impact of PDO and La-Niña-like decadal cooling
The internal variability in the climate system help to circulate heat into deeper levels of the ocean, but can also strongly release heat into the atmosphere during the El Nino part of the ENSO cycle (See Balmaseda et al 2013) or the positive phase of the Pacific Decadal Oscillation. in recent years we have had strong La-Niña cooling (Australia recording the wettest 2 year period on record). Read more about What ocean heating reveals about global warming in a blog post on Real Climate Blog by Stefan Rahmstorf.
Kevin Trenberth and John T. Fasullo (2013) argue in An apparent hiatus in global warming? that a switch to a negative phase of the Pacific Decadal Oscillation (PDO) in 1999 played an important role, "natural decadal variability modulates the rate of change of global surface temperatures while sea-level rise is more relentless. Global warming has not stopped; it is merely manifested in different ways."
The picture emerging is one where the positive phase of the PDO from 1976 to 1998 enhanced the surface warming somewhat by reducing the amount of heat sequestered by the deep ocean, while the negative phase of the PDO is one where more heat gets deposited at greater depths, contributing to the overall warming of the oceans but cooling the surface somewhat. The Pacific Ocean appears to account for the majority of the decadal variability [Chen et al., 2008]. Nevertheless, the events in the Pacific undoubtedly also affect the Atlantic, Indian, and Southern Oceans as the system acts collectively to equilibrate to these changes in the flow of energy.
Figure 8. The Pacific Decadal Oscillation based on an EOF analysis of SST anomalies with the global mean removed from 1900 to May 2013 in the 20∘ N–70∘ N and 110∘ E–100∘ W region of the North Pacific, which explains 25% of the variance. The principal component time series, given below in normalized units, is regressed on global SSTs to give the map above. The black curve is a 61 month running average. Source: Trenberth and Fasullo (2013)
Figure 6. The NOAA global mean 12 month running mean surface temperatures are given relative to 1901–2000 along with a linear trend fit. Marked on the graph are the El Nino (buff ) and La Nina (sky blue) periods as defined by NOAA’s ONI, based on the Nino 3.4 SST anomalies, as given in the lower panel relative to a base period of 1950–1979. Source: Trenberth and Fasullo (2013)
A study by Kosaka and Xie (2013) recently published in Nature argues that surface cooling in equatorial Pacific is driving the decadal pause in global temperature rise. Recent La Nina's are suppressing global average temperatures during the northern winter, but allowing temperatures to rise unabated in the northern summer. They say in the study abstract of Recent global-warming hiatus tied to equatorial Pacific surface cooling:
"Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase."
In 1998, the start of this period in question, was an extremely strong El Nino year which spiked atmospheric temperatures, and was followed by the start of a negative phase of the Pacific Decadal oscillation which acted to bury atmospheric warmth in deeper ocean layers. Sceptics cherry pick this year to start the period specifically to show the lack of change in the trend line. Tamino shows us statistically exactly what has been happening with Global Temperature: the Post-1998 Surprise.
Underestimation of temperatures by HadCRUT4 dataset
Another consideration, raised by Cowtan and Way (2013) study - Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends - is that atmospheric temperature measurements by HADCrut4 are not fully taking into account the rapid and extensive warming in the Arctic. The lack of estimation of temperatures in areas without observational data has lead to a bias in the HadCRUT4 dataset.
Watch a short presentation of the Cowtan and Way study below or read Kevin Cowtan's poster that explains the study. Stefan Rahmstorf discusses the implications of the study at the Real Climate Blog: Global Warming Since 1997 Underestimated by Half
Sources:
- IPCC Working Group 1 Climate Change 2013: The Physical Science - Summary for Policymakers, 27 September 2013.
- Tamino, 30 January 2014 - Global Temperature: the Post-1998 Surprise
- Cowtan and Way (2013), Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends (abstract - Full paper available)
- Kosaka, Y. & Xie, S.-P. Nature http://dx.doi.org/10.1038/nature12534 (2013)- Recent global-warming hiatus tied to equatorial Pacific surface cooling (abstract).
- Kevin E. Trenberth, John T. Fasullo, Earth's Future, 5 December 2013, An apparent hiatus in global warming? (abstract - Open access) DOI: 10.1002/2013EF000165
- Lead image of Dr Scott Power, principal research scientist of the Australian Bureau of Meteorology and IPCC AR5 Coordinating Lead Author of Chapter 11 at a Public Forum in Melbourne, 3 October 2013
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