What evidence is there for the hockey stick?

What The Science Says:
Recent studies agree that recent global temperatures are unprecedented in the last 1000 years.

Climate Myth: Hockey stick is broken
“In 2003 Professor McKitrick teamed with a Canadian engineer, Steve McIntyre, in attempting to replicate the chart and finally debunked it as statistical nonsense.  They revealed how the chart was derived from "collation errors, unjustified truncation or extrapolation of source data, obsolete data, incorrect principal component calculations, geographical mislocations and other serious defects" -- substantially affecting the temperature index.” (John McLaughlin)

At a glance

The Hockey Stick is a historic graph dating back to a paper published in 1999. It showed Northern Hemisphere temperature variations over the near-thousand year period from 1000-1998. The 'blade' of the stick represented the rapid warming of the late 20th Century. It has an iconic status, both in climate science and in the murky world of science-misinformation, where, naturally, it is despised by all and sundry.

Objections to the Hockey Stick are varied but mostly focussed on the stick's long handle and the data that represents. Obviously, during the centuries going back to 1000, reliable temperature measurements are not available. Fortunately for science, there are things that lived through that long time, such as certain very old trees. They record in the rings of their wood an indication of temperatures, year on year. Gardeners and farmers talk about good and bad growing years and it’s the same for natural systems. For example, cold dry periods make for narrow and densely-packed tree-rings whereas warmer, wetter times lead to more widely-spaced ones.

Importantly, today there are a great many effective past climate indicators, known as proxies because they act in place of thermometers. Because there's a range of indicators, the results from each one can be cross-checked against one another. If a new proxy is any good, its data should agree with that from the other, established ones.

Proxy datasets contain more uncertainty than directly measured temperatures. Everyone knows that. That does not mean they are useless, far from it. Cross-checking means poor data can readily be identified and investigated.

Finally, it's 24 years since the Hockey Stick graph was published. Since then, work on developing and refining the best proxies has been relentless. Better, longer temperature reconstructions have become possible. At the same time, global temperatures have continued to rise. In any of the observation-based records of surface temperature, all of the eight warmest years have been since 2015.

Please use this form to provide feedback about this new "At a glance" section, which was updated on May 27, 2023 to improve its readability. Read a more technical version below or dig deeper via the tabs above!

Further details

Reliable observational temperature records only go back so far in time – in the UK back to 1850 and in the USA to 1880, for example. So how do we find out about conditions going further back, hundreds, thousands or even millions of years into the past? We use proxies.

Proxies are things whose measurable properties are affected in certain, well-defined and understood ways by variations in temperature and other climatic parameters. Although the most well-known proxy work was undertaken by studying the rings of ancient trees, many other things have since shown usefulness in this field. They include data from ice-cores, marine and lake sediments and the fossils they contain, corals, mountain glaciers: as time goes by more and more things have shown themselves to be useful over a variety of time-spans. Armed with such tools, the paleoclimatologist can thereby reconstruct climatic conditions in ancient times, just as the paleontologist can reconstruct ancient ecosystems, from data preserved in the rocks.

By 1999, confidence in paleoclimate proxy data was sufficient to link these ancient records to modern observations and this was done in a famous paper by Michael Mann and colleagues (Mann et al. 1999), showing that global temperature gradually cooled over the last 1000 years, but then rose sharply, beginning in the 20th Century. The shape of the graph (Figure 1) therefore looked like a hockey stick lying flat on the ground with its blade pointing upwards.

Figure 1: Northern Hemisphere temperature changes estimated from various proxy records shown in blue (Mann et al. 1999). Instrumental data shown in red. Note the large uncertainty (grey area) as you go further back in time.

Controversy, mostly of a manufactured nature, raged over the hockey-stick graph in the years following its publication: it became a symbolic focal point in the online 'climate-wars' that characterised the first two decades of the 21st Century, a time of often bitter battles and recriminations as misinformers attacked climate scientists in any way they could think of.

In the meantime, the expansion of things that were found to work as effective proxies continued apace, so that by the late 2010s we had learned a lot more about paleoclimate going right back through the glaciations and interglacials of the Quaternary and into the warmer late Cenozoic era. The paleoclimate record today stretches a long way back, through tens of millions of years.

Back to shorter time-spans though, and another visually-catching graphic was recently created by climate scientist Ed Hawkins, using blues for colder years, reds for warmer ones and whites for near-average times. Known as “Warming Stripes'', the initial 2018 graph represented temperatures over the past 200 years, but a more recent version (Figure 2) uses a wide range of reliable proxy data from an international collaboration of scientists, called Past Global Changes 2K (PAGES2K), because it covers the past 2,000 years.

Warming Stripes past 2000 years

Figure 2: Warming Stripes based on PAGES2k (and HadCRUT4.6 for 2001-2019). Source: Ed Hawkins' Climate Lab Book

Warming Stripes is a visually striking graphic due in large part to its simplicity. It's like the Hockey Stick but with 20 years more scientific progress included. But like the Hockey Stick, it confirms the original findings: that the rate of recent warming is very steep in contrast to anything in the past two millennia.

Of course, as one would entirely expect, some crude attempts have been made to doctor Warming Stripes, but they never stand up to the level of scrutiny that scientists apply to their datasets. A graphic circulated in 2019 is one such example. For some unknown reason, its author left off the period 2007-2019, despite the temperature data being readily available. Could it have been because the warmest years on record occurred during this period? You tell us.

Furthermore, an in-depth examination of the graphic in question in a CBS News article shows that in fact it had been put together by crude copying and pasting in Photoshop or a similar application, so badly in fact that the edges of the pasted sections are clearly visible standing out from the top of the graphic. Like all climate misinformation, it just doesn't stand up to scrutiny.

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