"The trouble with the world is not that people know too little, it's that they know so many things that just aren't so."
-- Mark Twain
Climate Change Caused by Humans
Global Cooling?

Has Global Warming Ended?

Skeptics of the current global warming now refer to the period between 1998 and 2008 and claim that global warming has ended. Some go one step further and claim that global cooling has begun. Of course, the observed data shows that this is nonsense. Figures 5.2 and 5.3 (Sato, 2010) show global temperature changes measured by surface stations and also by satellite. The increasing trend in temperature certainly debunks claims that the world is experiencing global cooling.

Global temperatures from weather stations
Figure 5.2: Global temperature change measured by weather stations

Global temperatures over land and ocean
Figure 5.3: Global temperature change measured by satellite

According to NOAA climate monitoring chief Deke Arndt (Romm, 2009):

The last 10 years are the warmest 10-year period of the modern record. Even if you analyze the trend during that 10 years, the trend is actually positive, which means warming.

20 of the warmest years on record have occurred in the past 25 years. The warmest year globally was 2005 with the years 2009, 2007, 2006, 2003, 2002, and 1998 all tied for 2nd within statistical certainty. (Hansen et al., 2010) The warmest decade has been the 2000s, and each of the past three decades has been warmer than the decade before and each set records at their end. The odds of this being a natural occurrence are estimated to be one in a billion! (Schmidt and Wolfe, 2009)

Further signs of this warming trend can be seen in the Northern Hemisphere Sea Ice Extent from the National Snow and Ice Data Center. As the graph in the hyperlink shows, sea ice extent has been dramatically reduced since 1979.

So why do some climate contrarians claim that there is global cooling in the face of this data? It boils down to "cherry-picking" data to intentionally misinform the public. Figure 5.4 (Hausfather, 2008) shows global surface and lower troposphere monthly mean anomalies relative to the 1979-1998 mean temperature. The data is from GISS, HadCRU, RSS, and UAH ranging from January 1979 to February 2008. GISS, HadCRU, RSS, and UAH represent the four organizations that publish online the global average temperature estimates. Many experts believe that GISS data is the preferred set because it is the only set that uses all regions of the globe. HadCRU, RSS, and UAH do not include polar regions.

No Cooling since 1998
Figure 5.4: Global surface and lower troposphere monthly mean anomalies relative to the 1979-1998 mean temperature.

When one considers monthly mean temperature anomalies from 1998 to 2008 it might “appear” that there is a flat line or global cooling. Keep in mind two important points:

  1. 1998 was a strong El Niño year which caused a very warm signal and 2008 had a strong La Niña which caused a very cool signal.
  2. These are ANOMALIES not actual temperatures. These are values compared to 1979 to 1998 means.
One will notice that MOST of the time between 1998 and 2008 the anomalies are POSITIVE which means most of the time the planet was warmer than 1979-1998 means. GISS and HadCRU both show a warming trend of 0.16 oC per decade from 1979 to February 2008. RSS shows a warming trend of 0.18 oC per decade over the same period, while UAH shows a warming trend of 0.14 oC. One cannot cherry pick the endpoints to make the graph that they “want to see.” The global warming critics choose 1998-2008 because they know that using these endpoints makes the trend look flat or negative!

Figure 5.5 (Clark, 2009) focuses on the years 1998 to 2008 which is the time period used by those that wish to deceive the general public into thinking that global cooling is occurring.

Temperature trends 1998 to 2008
Figure 5.5: Global surface and lower troposphere monthly mean anomalies and linear trends between 1998 and 2008

Three of the four global average temperatures indeed are decreasing in their trends (although the actual global mean temperatures are still warmer than the previous decades).

Figure 5.6 (Clark, 2009) focuses on the years 1999 to 2009.

Temperature trends 1999 to 2009
Figure 5.6: Global surface and lower troposphere monthly mean anomalies and linear trends between 1999 and 2009

Simply by shifting our starting point by one year, all four global average temperatures are increasing in their trends! The point made here is that if one cherry-picks a small subset of the data, one can make just about any claim with a nice plot to back it up. The correct way to view global temperature trends is to look at ALL of the data. Figure 5.7 (Clark, 2009) shows the global average temperatures along with trends from 1880 to present. (Note: UAH and RSS data does not exist before 1980)

Temperature trends 1880 to 2009
Figure 5.7: Global surface and lower troposphere monthly mean anomalies and linear trends since 1880

It is quite obvious that global temperatures have been increasing since 1880 and at a faster rate in the past two decades.

Still not sure about short-term trend (weather) vs. long-term trend (climate)? Play around with this wonderful Java applet developed by Colin Sharples of CTG Software Ltd:

The slider at the bottom starts at 30 years — the standard period for climate stats. Choose between data series with the button at the top. Blue lines show negative trends, red ones positive trends, calculated for each successive 30 year period. When one slides the bar to shorter intervals such as ten years, there will be more blue trends. However, ten years is not going to separate the noise from the signal but thirty years or more does.

Tamino (2009) has a very good blog post about global temperature trends since the 1970s called Riddle me this.... Many skeptics claim that the warming predicted by scientists did not happen in the 2000s. The data shows otherwise!

GISS Expected Trend
Figure 5.8: NASA GISS for the time span 1975 to 2000 (Ibid)

The solid red line is the trend using linear regression and it is extended as a dashed line to indicate this same trend into the future to the year 2009. The top and bottom dashed lines represent the bounds of two standard deviations where most data points are likely to fall. Figure 5.9 below shows what actually happened in the 2000s. So where is the cooling?

GISS Actual Trend
Figure 5.9: NASA GISS for the time span 1975 to 2009 (Ibid)

Prefer to use satellite-derived lower troposphere global temperatures so that actual thermometers are not being used (avoids any claimed UHI effect)? RSS and UAH are the two choices and UAH is run by two of the leading skeptics: Christy and Spencer. What did the UAH pre-2000 trend do in the 2000s? See figure 5.10 below. Yes, it continued to warm up!

UAH Actual Trend
Figure 5.10: UAH for the time span 1979 to 2009 (Ibid)

Zeke Hausfather from the Yale Forum on Climate Change & the Media has a similar analysis using HadCRU data. See: Claims of a Decade of Cooling Refuted By Analysis Showing It Warmest by Fair Margin.

Much of the heat that is delivered by the sun is stored in the Earth's oceans while only a fraction of this heat is stored in the atmosphere. Therefore, a change in the heat stored in the ocean is a better indicator of climate change than changes in atmospheric heat. Figures 7.2 and 7.3 (Richardson et al., 2009) displayed on my Modern Day Climate Change page clearly show that the oceans have warmed significantly in recent years and the trend is 50% greater than that reported by the IPCC in 2007. There is certainly no global cooling in the oceans!

To summarize the multiple lines of evidence that show there certainly is no global cooling:

  1. 20 of the warmest years on record have occurred in the past 25 years. The warmest year globally was 2005 with the years 2009, 2007, 2006, 2003, 2002, and 1998 all tied for 2nd within statistical certainty. (Hansen et al., 2010) The warmest decade has been the 2000s, and each of the past three decades has been warmer than the decade before and each set records at their end. The odds of this being a natural occurrence are estimated to be one in a billion!
  2. Temperature data from 1850 to present shows that there has been an increasing trend and the rate of warming has increased rapidly in the past few decades.
  3. Surface temperatures north of latitude 60o are warming at an accelerated rate in the past few decades.
  4. The Arctic was experiencing long-term cooling in the past 2000 years according to Milankovitch cycles until very recently. The cooling trend was reversed during the 20th century, with four of the five warmest decades of the 2000-year-long reconstruction occurring between 1950 and 2000.
  5. Sea ice extent has been dramatically reduced since 1979.
  6. Since measurements began in 2004, there has been a dramatic decrease in sea ice thickness.
  7. Greenland is losing ice mass and the rate is accelerating.
  8. Antarctica is losing ice mass and the rate is accelerating.
  9. The average mass balance of the glaciers with available long-term observation series around the world continues to decrease.
  10. 90% of worldwide glaciers are retreating.
  11. Much of the heat that is delivered by the sun is stored in the Earth's oceans while only a fraction of this heat is stored in the atmosphere. Therefore, a change in the heat stored in the ocean is a better indicator of climate change than changes in atmospheric heat. The oceans have warmed significantly in recent years.
  12. The oceans are taking in almost all of the excess heat since the 1970s which underscores the point that ocean heat content is a better indicator of global warming than atmospheric temperatures.

Global CoolingPeter Sinclair's Climate Crock of the Week: 1998 Revisited
If you prefer to watch a video explaining much of what appears on this Web page, please watch this video.
Global CoolingPeter Sinclair's Climate Crock of the Week: Party Like it's 1998
and this video.

Here is a more technical analysis of why global temperatures have not "cooled since 1998" nor "cooled since 2001" as some global warming critics claim: Embarrassing Questions from the Open Mind Blog.

Tamino (2009) also shows that at least 15 years of data is required for any statisitically significant trend to be determined. Using any period shorter than 15 years will result in the "noise" dominating the "signal".

Temperature trends - pick a timescale, any timescale! by Paul Clark, a British software developer, also shows the dangers of choosing time scales that are too short to determine a real trend.

Next: Climate Models & Accuracy



Scott A. Mandia
Professor - Physical Sciences
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Last updated: 06/09/10