Does this graph show climate-change predictions don't meet observations?

Question

Dr. Roy Spencer, a climate change contrarian, posted this graph on his blog claiming that observed temperature trends are much lower than what climate models predict:

• Is Spencer misrepresenting what the mainstream models predict? If so, how?
• Is the observed temperature trend on the graph representative of the data sets used by climate scientists?

I looked at the National Climate Data Center web-site, but there are so many different data sets, and I hardly know enough about climate science to know which ones to look at and how to interpret them.

I looked at Skeptical Science's article about Roy Spencer, and found plenty of articles concerning his arguments and his 2010 paper in Remote Sensing, but found nothing concerning this more recent post.

This blog comment says RCP 8.5 -- the highest RCP -- was used. But I have no idea what this means or how it invalidates or misrepresents the output.

Update: I found this here which makes it look like there is little difference in end CO2 concentration for the different RCPs at least till 2025.

What is Spencer doing to get this graph? Is he cherry picking climate data? Misrepresenting the model's predictions, or is it accurate?

Answer 1

There are at least three issues here which contribute to the large differences which Spencer describes.

First, there's a highly misleading aspect to the chart. People not familiar with the subject, or who just glance at the chart quickly might conclude that experimental data show essentially no change even though all the climate models are predicting significant temperature increases - which would indicate that global warming isn't really occurring. This is not what the chart shows. In fact, Spencer has selected temperature predictions and measurements just for a specific part of the atmosphere, over a specific part of the earth:

• The middle of the lower atmosphere (the mid-troposphere)
• Near the equator (between 20 degrees north and south of the equator).

He did not make it clear in the post that he's focusing on this single value. While people who are knowledgeable in the field will notice what's being plotted, he's implicitly suggesting that predicted temperature increases are consistently much higher than observations, which is not true. Without speculating on whether this is intentional, he is certainly aware that many people will see (and share) the chart as-is, without understanding what it's truly saying.

A more global comparison clearly shows the models are working quite well:

Second, there is some truth in the chart. There is a known discrepancy between tropical tropospheric temperature predictions and observations. This has been recognized for years, so it's nothing new. There's some discussion here which explains the issue and sums it up by saying it's not clear whether it's caused by inaccuracies in the models, inaccuracies in the temperature data, or some combination of the two:

The apparent model-observational difference for tropical upper tropospheric warming represents an important problem, but it is not clear whether the difference is a result of common biases in GCMs, biases in observational datasets, or both.

Third, there's some clear cherry-picking:

• He has focused on a single temperature - the tropical mid-troposphere. That means he's looking at atmospheric temperatures in a particular range of elevations, within a particular range of latitudes. Thus, he's ignoring all other temperature predictions and measurements, which present a much different picture as shown in the chart above.

• He has chosen not to illustrate areas where the consensus model predictions are understating the severity of climate change, for example polar ice melting or global sea level rise - which are occurring much faster than models predict.

source

source (pdf file)

• He has used the Representative Concentration Pathway (RCP) 8.5 predictions from all the models. There are 4 standard CO2 emissions scenarios used in climate modeling, all of which are considered possible. Here is a good summary of what RCP means Of the four, 8.5 represents the highest CO2 emissions, thus the greatest climate change:

The worst case scenario - RCP8.5 - assumes more or less unabated emissions.

While the difference isn't particularly large in the early years, it is clear that he intentionally selected the RCP value which results in the greatest predicted increases because that will maximize the difference between predictions and observations

Answer 2

Spencer's chart has serious problems but other analysis says the answer is that most models are currently overestimating warming.

The debate has been significantly obscured because the majority of contributors to it have chosen what appear to be poor ways of testing the claim with actual data (which is not, in principle, hard as the key data from models and observations are accessible).

Climate skeptics have used time series plots to compare models to observations but these are beset by bad choices of the normalisations for temperature anomalies and are a poor way to compare the underlying patterns due to the noisy nature of temperature time series.

See the claims Spencer makes here WattUpWithThat and a good critique of the errors here.

But there are ways of simplifying the comparison that don't suffer from these defects and these have been done by people who don't seem to have the same partisan desire to reach a particular conclusion.

This picture from a Nature article is the clearest analysis that avoids the various obfuscations possible when looking at noisy time series data:

The choice made by these authors (and it is worth noting that they are not skeptics and published in a prestigious mainstream journal) is to compare just the average extent of warming over a period rather than the noisy time series. This appears to avoid some of the choices that make the time series comparison so obfuscating and controversial. Their result seems to show that most models really do overestimate the actual amount of warming over recent years. In their words:

Recent observed global warming is significantly less than that simulated by climate models.

In somewhat more detail the authors talk about the statistics like this (my highlights):

The evidence, therefore, indicates that the current generation of climate models (when run as a group, with the CMIP5 prescribed forcings) do not reproduce the observed global warming over the past 20 years, or the slowdown in global warming over the past fifteen years. This interpretation is supported by statistical tests of the null hypothesis that the observed and model mean trends are equal, assuming that either: (1) the models are exchangeable with each other (that is, the ‘truth plus error’ view); or (2) the models are exchangeable with each other and with the observations (see Supplementary Information). Differences between observed and simulated 20-year trends have p values (Supplementary Information) that drop to close to zero by 1993–2012 under assumption (1) and to 0.04 under assumption (2) (Fig. 2c). Here we note that the smaller the p value is, the stronger the evidence against the null hypothesis. On this basis, the rarity of the 1993–2012 trend difference under assumption (1) is obvious. Under assumption (2), this implies that such an inconsistency is only expected to occur by chance once in 500 years, if 20-year periods are considered statistically independent. Similar results apply to trends for 1998–2012. In conclusion, we reject the null hypothesis that the observed and model mean trends are equal.

The conclusion is that most models (or, perhaps, most results from the ensemble of models) predict more warming that has actually happened. Whether this is a significant problems for models will become more obvious in the next few years as observational data accumulates.