ericgrimsrud

In the last issue of Science, a significant extension in our knowledge of the changes in the Earth’s average temperature since the last glacial period has been provided. Prior to this report, temperature changes over the last millennia up to the present time had been reported by several research groups. Those measurement led to what became known as the “hockey stick” shown below.

While the uncertainty of those proxy temperature measurements of several centuries ago were relatively large , those of the more recent measurements were much better and these clearly indicated a rapid increase in global temperatures in recent decades.

Marcott et al., Science (2013) have now merged the above historic temperature record with the additional proxy measurements going back approximately ten millennia, as shown by the following figure.

These additional measurements put the temperature changes of the last millennia into greater perspective and show that the present average temperature of the Earth is roughly the same as the highest temperatures previously observed over the entire Holocene. They also show that the rate of our present temperature rise is far greater than any ever observed during the Holocene.
For even greater perspective, these new measurements have been combined in the last figure shown below with other measurements going still deeper in time to the final stages of the last glacial period about 20 millennia ago (green line).  In this figure our expectations of future temperature increases extending up to the year 2100 have also been included and are shown in this figure by the red line:

This figure provides a complete perspective of the temperature changes that have occurred during the period of time slightly before and after human civilizations first began in about 6,000 BC and before and after fossil fuels first began to be used extensively for energy production in about 1850 AD. The pattern of change thereby observed over the entire Holocene does indeed resemble a wheelchair.

Bottom line: The sharp rise in measured temperatures over the Industrial Age clearly suggests that something unprecedented over the entire Holocene is now occurring. Moreover, calculations of the expected effect of increased levels of greenhouse gases is in agreement with these recent temperature rises and this, in turn, lends support to the expected effects on temperature of business-as-usual means of energy production if they continue throughout the 21st century.

As they say, that last picture is indeed worth a thousand words, so need I say more? I don’t think so – other than to remind everyone that a temperature rise of 3 degrees Centigrade above present levels is thought to be entirely incompatible with existing forms of human civilization. In addition, a temperature rise of that magnitude is thought to be likely to set in motion huge additional emissions of naturally accumulated reservoirs of methane and carbon dioxide – which would very likely cause additional “run away” and irreversible temperature increases well beyond those predicted by the red line.