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It's the sun
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What ended the Little Ice Age?

The skeptic argument...

"The global temperature has been rising at a steady trend rate of 0.5°C per century since the end of the little ice age in the 1700s (when the Thames River would freeze over every winter; the last time it froze over was 1804). The IPCC blames human emissions of carbon dioxide for the last warming. But by general consensus human emissions of carbon dioxide have only been large enough to be significant since 1940—yet the warming trend was in place for well over a century before that."  (David Evans)

What the science says...

The main driver of the warming from the Little Ice Age to 1940 was the warming sun with a small contribution from volcanic activity. However, solar activity leveled off after 1940 and the net influence from sun and volcano since 1940 has been slight cooling. Greenhouse gases have been the main contributor of warming since 1970.

The Little Ice Age was a cooler period spanning the 16th to the 19th century. The river Thames often froze over. The Norse colonies in Greenland were unable to survive the harsh winters. After 1850, temperatures began to rise. But man-made CO2 emissions in the late 19th century were a fraction of current levels. Did human activity take us out of the Little Ice Age? Were there other factors? And what does it mean for current warming? This question is addressed in an analysis which examines the various factors that drove climate since the 19th Century (Meehl 2004).

Climate simulations were run using two natural factors that drive climate - volcanoes and the sun. Volcanic eruptions cool global temperature for a few years after eruption. A drop in volcanic activity after 1915 contributed slight warming in the early 20th Century. However, the greater contributor to warming from 1880 came from the sun which steadily warmed up to the 1940s. When the two factors are combined, they account well for the warming from 1880 to 1940. However, the contribution from sun and volcanoes to global temperature since the 1940s has been a slight cooling effect.


Figure 1: Climate model results from natural forcings compared to observations (black line). The red line is the average of the four-member ensemble. The pink shading is the model range. The blue line is the ensemble mean and the light blue shading is the ensemble range.

To calculate the human influence on climate, three forcings were considered: ozone, sulfate aerosols and man-made greenhouse gases. Sulfate aerosols have a cooling effect, growing stronger after around 1950. Changes in ozone produced a slight warming response. The strongest effect is from greenhouse gases which produced slow warming in the early 20th Century, then accelerated in the 1970s. When all anthropogenic forcings are combined, they show little temperature response from 1880 to to the 1960s. They do a poor job of explaining the warming from 1880 but a very good job of capturing late 20th Century warming.


Figure 2: Climate model results from anthropogenic forcings compared to observations (black line).

When natural and anthropogenic forcings are combined, they show close correspondance with global temperature. From this analysis, Meehl concludes that the warming from 1880 to the mid-20th Century was largely natural with the sun being the main contributor. Late 20th Century warming after 1970 is mainly due to man-made influence primarily from greenhouse gases.


Figure 3: Climate model results from natural + anthropogenic forcings compared to observations (black line).

This analysis is a useful reminder that CO2 is not the only driver of climate. To end the Little Ice Age, the sun did most of the early heavy lifting. When the solar contribution flattened out in the mid-20th century, humanity took the baton and we've been running with it ever since. Meehl 2004 is also confirmation that past climate change tells us how sensitive climate is to radiative forcing. The climate that responded to the forcing from the warming sun in the early 20th century is the same climate that is now responding to the forcing from rising greenhouse gases.

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Comments

Comments 1 to 13:

  1. Mr. Cook:

    I think the whole flaw in the climate change argument can be expressed examining the words you have just used when saying "What the science really says":

    "The main driver of the warming from the Little Ice Age to 1940 was the warming sun with a small contribution from volcanic activity. However, solar activity leveled off after 1940 and the net influence from sun and volcano since 1940 has been slight cooling."

    OK, fine. While I don't necessarily agree with the sun portion (see Reply to Lockwood and Fröhlich - The persistent role of the Sun in climate forcing here - http://www.spacecenter.dk/publications/scientific-report-series/Scient_No._3.pdf/view), let's say everything you have said there is true. Even so, that does NOT automatically mean:

    "Greenhouse gases have been the main contributor of warming since 1970."

    Do we have any direct proof of that? Do we know exactly how much radiative forcing the greenhouse gases we emit produce? And do we know how much they produce when within the extremely complex climate system, as opposed to within laboratory conditions? Or is it just an assumption, considering we have exhausted all the possible natural causes that we can think of?

    I think that there may be other natural causes (maybe even ones we have not yet discovered) causing this kind of warming, at least to a certain extent. Syun-Ichi Akasofu here (http://people.iarc.uaf.edu/~sakasofu/little_ice_age.php) wrote about the possibility that much of the current warming could be simplified down to a natural 0.5 degrees C linear trend, with superimposed fluctuations and oscillations.

    Most importantly, he also notices that global warming has essentially stopped since 2000. This lack of warming does not agree with IPCC predictions. Instead it gives more credibility to this theory, as it could be explained as the most recent oscillation winding down and continuing on the 0.5 degrees C linear trend.
    Response: The direct proof of the radiative forcing from rising CO2 is explored in the empirical evidence for an enhanced greenhouse effect.

    Re Akasofu's assertion that "global warming has essentially stopped since 2000", presumably, this is based on the HadCRUT surface temperature record which omits areas of the globe of extreme warming in recent years. A more comprehensive analysis of the Earth's energy imbalance finds the planet continued to accumulate heat past 2000 right up to the end of 2008 (where the analysis ends). Global warming has not stopped.
  2. No, I'm not just looking at the CRU measurements. Take a look at NASA's Goddard Institute for Space Studies measuements (http://data.giss.nasa.gov/gistemp/graphs/). They also show global warming slowing down and almost stopping at about 2000.

    At the very least, if global warming has not stopped, it has definitely slowed down quite a bit, against what the IPCC has predicted.
  3. Sorry, ignore the part about GISS. Check out the University of Alabama in Huntsville's measurements: http://www.nationalpost.com/893554.bin
  4. michaelkourlas,
    global warming didn't stop nor slowed down, at least if you mean the trend.
    Maybe temperature did, but it has the bad habit of going up and down in the short run; it always did and presumably will continue to do so. And the neither the IPCC nor the climatologists ever said it will not.
  5. I understand that the overall global warming trend has gone on for hundreds of years since the little ice age. I am not disputing that. What I am disputing is how much warming there has been since the 1980s, which is the warming we would be responsible for. We only started emitting massive amounts of CO2 in 1945 and the first time the temperature started rising after that was in the late 1970s and early 1980s.

    Thus, the problem is that all the IPCC looks at to make its predictions and recommendations and such is one 30 year trend from 1980-present. 30 years is not really enough to completely determine whether or not humans are having a sizable impact on the climate system in the first place, but if there is a possibility that last 10 years of that series do not conform to the predictions that the IPCC have made... it really throws the IPCC's predictions into doubt.
  6. michaelkourlas,
    if 30 years are not enough to determine a trend i do not understand how 10 years can be enough. And it's not just 30 years, although smaller the contribution of ghg is sizeble even in the first half of the last century.

    Also, that the models did not predict the last 10 years is a mith, not least because they never attempted to predict them. I'd suggest to read how meaningfull comparison should be done.
  7. I'm not trying to make predictions off of a 10 year trend. All I am saying is that 30 years, 10 years of which do not follow predicted trends, is not enough to establish that we are having a large effect on climate.

    In response to "the models did not predict the last 10 years...because they never attempted to predict them", the link you gave actually shows the measurements not following the IPCC predictions.
  8. michaelkourlas,
    if i got it right, 30 years are enough to establish a trend. If this is true, then the last 10 years are well inside the measured variability and cannot falsify the trend in any way.
    But, given that you think that fig. 1 in the link i posted before "shows the measurements not following the IPCC predictions" you problably can say almost anything you like. The numbers tell a different story, though.
  9. One can establish a trend from just a few years. The question is whether or not that trend is enough to justify spending large amounts of money and devastating certain economic sectors in the name of 'saving humanity'. I don't think 30 years of data, 10 of which show a decline in temperature, is sufficient for this.

    As for the IPCC predictions, maybe I'm making a mistake, but I thought that the black line was the IPCC trend, and the blue and red lines showing temperature have deviated from that line since 2005. That's at least 1/6th of the 30 year trend off course.
  10. This is a good site for examining IPCC predictions and their failures.
  11. Also look here for the natural vs anthropogenic prediction information (http://www.appinsys.com/GlobalWarming/PredictionFromCycles.htm)
  12. michaelkourlas wrote "maybe I'm making a mistake, but I thought that the black line was the IPCC trend."

    Michael, you are indeed making a mistake, as Riccardo tried to explain to you. The IPCC prediction is the entire gray area in that figure. The black line is merely the mean, which is merely the most probable point-by-point portion--the central tendency of the prediction, not the range of the prediction. The range is the gray area. Nobody, least of all the IPCC, expects the actual values to always fall exactly on the line. Nobody even expects the actual values to always fall within the gray area. Instead the expectation is that sometimes the actual values will be above the line and sometimes below the line, but on average they will fall more or less the same amount of time above as below the line, and almost all of the time they will fall within the gray area.

    When you pick out the most recent four years as being below the black line, you are conveniently ignoring the several periods before that being above the line. Oh, but then you could point to the previous couple years being below the line. But then you'd be ignoring the years before that being far above the line.... And so on. If you play that game all the way back to the start of the graph, you see that on average the actual values spend nearly as much time above as below the line, and always inside the gray area.

    There are formal, systematic ways of doing the above analysis. They were not invented for climatology. They have been used for many decades in many different fields of science and technology. They are being applied to climatology in exactly the same way. You can start to learn about them by reading Tamino's post "How Long?"
  13. michaelkourlas,
    you can surely use just a few years to calculate a trend but you can not establish it this way; it would be just a mathematical exercise. Physics, and climate science as well, uses mathematics as a tool but they also give a meaning to the numbers.
    When the data points have a "noise" of about 0.2 °C and a trend of about 0.17 °C/decade even common sense should convince you that it makes no sense at all to use just 7 years to calculate a meaningful trend.

    If you really want to understand what is going on with our climate, it would be a good idea not to use "blind" google searches. You know, the internet is a great tool, but you can find almost anything you want. Given that we not always have the knowledge to state the credibility of a source by ourselves, an a priori reasoned choice is mandatory. Or anyone can fool you.

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