The Holocene – the time since the end of the last glaciation – which has witnessed all of humanity’s recorded history and the rise and fall of civilisations – began only 11,700 years ago. It is a relatively warm period, but how warm was it at its warmest? What happened in the past informs current climate models placing the current global warming into perspective.
There have been contrasting views on the warmth of the Holocene. Marcott et al (2013) says 0.8°C higher than the pre-industrial period, Lui et al (2014) says 0.5°C cooler. In general, recent CMIP6 models fail to cast much light on the so-called Holocene temperature conundrum.
In a new review of available data Kaufman and Broadman (2023) find a relatively mild thermal maximum, but say more research is needed to understand slow-moving climate variability. They looked at a large variety of natural processes operating in many settings which the researchers say do not all point in the same direction. The temperature probably peaked about 6,500 years ago about 0.5°C higher than when compared to 1850 – 1900, the warmth chiefly occurring in middle to high latitudes of the northern hemisphere. This is a curious finding as today’s more rapid warming also occurs predominately at high northern latitudes – Arctic Amplification.
Global Temperature relative to 1850-1900, from Kaufman and Broadman.
Europe’s Summer Hydroclimate
Looking less farther back Freund et al (2023) use tree ring data to examine the European summer hydroclimate back to 1600 and make some interesting findings.
The current analysis utilises widespread, long-term data sets spread over many centuries. Those familiar with the controversy of the “hockey stick” tree-ring analysis will know that tree-ring widths require statistical detrending, making it difficult to use them to reconstruct some aspects of low-frequency climate variability. Furthermore, tree-ring width data from European lowlands can display weak and ambiguous climate signals. In contrast, tree-ring stable isotopes are thought to be a more useful proxy as they can require less statistical processing,
One of the tree-rings used to establish Europe’s hydroclimate. Tree-rings allow precise dating into the past:The isotope ratios of carbon 12C to 13C and oxygen 18O to 16O in the cellulose of the tree trunk are indicators of drought or moisture at the time of tree growth.
There has been much debate about whether the European summer drought (2015–2019) was within the range of natural variability or is due to anthropogenic warming. Looking back at historical records suggests there has not been two consecutive summer droughts in central Europe in the previous 250 years. Also, an extensive tree-ring isotope record from the Czech Republic indicates that the recent drought was unprecedented in the last two millennia. Others have said that based on tree-ring records and reanalysis products, the recent drought is well within the range of natural variability is thus not unprecedented.
In the new study the researchers reconstructed European hydroclimate based on a network of tree-ring stable isotopes of oxygen and carbon ratios. The network combines up to 400-year long, annually resolved records of deciduous oaks from European lowlands and conifers from boreal and mountainous sites. This is the first time that stable isotope records from tree rings, with their so far untapped high climate reconstruction potential, are used to obtain a gridded spatial reconstruction of the European summer hydroclimate.
They find three distinct phases and the possible signature of solar activity. Firstly, the years 1600 – 1652 had a wetter climate most apparent in west-central and north-west Europe. Secondly, shortly after the start of the Maunder Minimum (a period of low solar activity) drier conditions began that lasted two centuries affecting the Mediterranean, East and northern Europe. By the end of the Little Ice Age, about 1875, another change occurred. Central, northern Europe and the Mediterranean show milder summer conditions that prevailed until the 1950s.
There is a trend towards drier conditions in the second half of the 20th century with 2003 and 2006 being the driest years on record which the researchers say is unusual through not unprecedented. As for the 2015 – 2019 drought most of Europe experienced an unprecedented drought with moisture conditions very much below average and in some parts the lowest on record. At the same time however Scotland and parts of Scandinavia as well as Greece and Turkey experienced unusual wet conditions. The researchers conclude that whilst it seems that 2015 – 19 drought was unprecedented there might be evidence of longer and more severe droughts.
Another recent research report has scientists analysing 1,000-year old ice from Greenland. The Greenland Ice Sheet has a central role in the global climate system sometimes described as a potential tipping point due to its sudden collapse.
Greenland Ice Cap temperature; something happened in 1800?
Greenland’s coastal regions are warming but the researchers point out that the imprint of global warming in the central part of the ice sheet is unclear due to a lack of long-term observations. Ice-core based reconstructions are ambiguous and cannot separate global warming forcing from natural variability.
The researchers produced an ice- core from central Greenland from which they are able to deduce temperatures between AD 1000 – 2011. They find that by 2011 it is 1.5°C +/- 0.4°C warmer than the 20th century leading them to conclude that the anthropomorphic influence has reached central and northern Greenland, although confirmation is clearly needed.