Slapped by the Hockey Stick - Summer on Spaceship Earth

The "hockey stick" diagram representing the change in global temperature over the last millennium received its fair share of scrutiny and criticism and more than its fair share of ridicule. Taking guidance from the National Academy of Sciences, a group of scientists recently published an update of their analysis of global temperature records. Their conclusion: the world is hotter right now than it has been in the last 2000 years.

Instruments for recording temperatures only became widespread in the mid 1800s. Thus, "instrumental" records that indicate global temperatures are available only the last 100 years or so. Michael E. Mann from Pennsylvania State University and his colleagues used 1209 "proxy" time series. Proxies are measurements of one variable that can be used to provide information about another variable. The classic example is tree rings. The width of the annual rings in a tree trunk depend, in part, on temperature. Mann and his colleagues used data from tree-ring, marine sediment, lake sediment, cave deposit, ice core, coral, and historical documentary time series. This database is significant extension of the database used earlier studies.

Mann and his colleagues use several methods, including new methods that make better use of the the information about spatial variability, to create alternative reconstructions of the temperature history of the last 2000 years.

Figure S15 from Mann et al. different colors show alternative reconstructions. The green line and shaded area is the best estimate, and the red line starting around 1850 is the instrumental data

Their main conclusion is that the warmth recorded in the Northern Hemisphere over the last decade exceeds the past 1300 years with a probability between 66% to 90%. Not certain, but what is?

They are less confident about the Southern Hemisphere because the relatively small number of available temperature proxies. Nevertheless recent warmth in the Southern Hemisphere is anomalous with respect the last 1500 years. There are simply not enough folks counting tree rings down south.

This kind of historical reconstruction is difficult and fraught with uncertainty, a word whch appears 31 times in the main article. Nevertheless we should take the conclusion very seriously. The alternative reconstructions generally agree with one another and fall mostly within the uncertainty envelope of the primary reconstruction.

Our world is hotter now than it has been in at least 2000 years, and it is getting hotter. How will we adapt to this change?

Global temperatures between about 1400 and 1960 were approximately 0.5C cooler than the 1961-1900 reference period. Plus there was the famous "Medieval Warm Period" centered around when the temperatures were some 0.2C warmer. What do we know about the effects of these changes on human societies? On agiculture? On fishing?

Even if we were to stop all emissions of greenhouse gases immediately, the globe will continue to warm. It will take many decades for global biogeochemical cycles remove these gasses. We need to prepare for more warming. The hockey stick should give all of as a slap shot.