Above, you can see an image from the paper by Marsicek et al., just appeared on Nature. It shows a reconstruction from pollen records of the temperatures of the past 10,000 year or so, the “Holocene,” for North America and Europe. Note the black squares, showing how fast temperatures have been growing during the past 50 years or so.
As all reconstructions of the past, this one has to be taken with some caution, but it fits well with the various “hockey sticks” that research continues to produce despite the attempts to discredit both the science and the scientists who work in this field. So, we can assume these results to be reasonably reliable. Then, we can note a few interesting things.
- What we call “civilization” arose and continued to exist during a period of relatively constant temperatures, that is, during the past 5000 years or so. During this period, the oscillations in the graph are never more than about half a degree. That’s probably not a coincidence. Agriculture and civilization come together and it is unlikely that agriculture could have been developed for wildly oscillating temperatures and rapidly varying climates
- Civilizations seem to grow and collapse because of internal factors – the fall of empires doesn’t seem to be correlated to climate change. For instance, you can look in the graph for the data corresponding to the fall of the Roman Empire, between 2000 and 1500 years ago. Temperatures are flat, at most cooling a little. It is a point that I already madeon the basis of another set of data specific for the region occupied by the Roman Empire. These more detailed data show a cooling period in Europe, but afterthe fall of the Empire.
- Some relatively intense oscillations in the curve appear at about 3000 years bp, which corresponds to the collapse of the Late Bronze Age civilization. This might support the interpretation byEric Clinewho sees climate as a concause of the collapse. Maybe, but can a whole civilization collapse abruptly as the result of a temperature change of just a couple of tenths of degrees? Personally, I tend to think the opposite. That is, the modest temperature change of the Late Bronze Age has been triggered by the collapse of the Mediterranean civilization of that time.
- Note how some much touted events of the past – for instance the “Medieval Warm Period” – appear as just minor perturbations in the curve. Overall, it seems that the effect of human activity on climate has been marginal until the age of fossil fuels.
- According to Ruddiman, the relative stability of the past 8000 years or so is the result of the release of greenhouse gases produced by human agriculture. This is the phenomenon which prevented the earth system to return to a new ice age. It is possible, but it seems to me at least unlikely that a system can be stabilized by two opposite strongperturbations (the other one is the effect of the Milankovitch oscillations)
- There is no obvious correlation of this long term trend with what we know of the Sun’s output. There has been a lot of speculation that the past temperature oscillations have been related to variations of the Sun’s output — the “Maunder Minimum” is an example of that. But if these variations have an effect, it is truly minimal. It can only be within the oscillations of the curve which don’t exceed a few tenths of degree.
- The increase in temperatures during the past 50 years or so has been simply stunning. In a sense, these sudden temperature changes are not unusual in the earth’s history (the problem for biological species is to survive them). But, in this case, it is so fast that it has probably no equals in the whole geological history of the planet. It is a disaster ongoing. Will civilization survive? Will humankind survive? Will anythingalive today survive? Who can say?
But don’t worry: we all know that this paper is part of the great conspiracy of the 97% of the world’s climate scientists. Fortunately, they have been debunked by a group of brave internet trolls, helped by friendly fossil fuel lobbyists.
The paper cited here is behind a paywall. If you have no access to it, write to me (ugo.bardi(twiggyingthing)unifi.it) and I’ll send you a copy.
Ugo Bardi teaches physical chemistry at the University of Florence, in Italy. He is interested in resource depletion, system dynamics modeling, climate science and renewable energy. Contact: ugo.bardi(whirlything)unifi.it . This article first published in his blog Cassandra’s Legacy