Introduction
One of the important aspects of global warming is its impact on weather extremes we are likely to experience in the future. We need to be able to investigate possible climate scenarios based on an expected range of greenhouse gas emissions based on human activities. These scenarios, in turn, will inform our efforts to develop a mix of strategies to both adapt to (where necessary) and to mitigate (preferably) global warming to the greatest extent possible. What follows discusses the tools available to investigate potential climate futures.
Tools for the Investigation
The tools necessary to estimate any future climate extremes are good climate models, computational hardware to run them, supercomputers either owned or in the “cloud”, and proxy data for air temperature and other important variables from past climates over the globe. What are proxies for temperatures? Examples include the fraction of a layer of coal made up of stable isotope carbon-13 (not radioactive 14C), the pollen found in lake or ocean sediments, the relative fraction of oxygen-18 (18O) to 16O contained in ice cores, fossilized pollen, and the fraction of stable isotopes of elements found in rocks. A good, detailed write up on climate proxies can be found in Wikipedia.
What We Know From the Past
The reconstruction of temperature proxies from earliest (Cambrian, beginning about 570 million years ago) to the current period (Holocene, 11,700 years ago to today) is found below, plotted from various studies since 2004 (Royer et al., 2004, Lisiecki and Raymo (2005), Hansen et al., 2013, and Marcott et al., 2013). The data are found in five linked panels:
- Paleozoic and Mesozoic (Cambrian through Cretaceous periods, 570 to 75 million years ago)
- Three optimal climate periods with one severe glacial period 300 million years ago and a second less severe cool period about 170-140 million years ago
- Paleogene (Paleocene through Oligocene, 75 to 5 million years ago)
- Started with one of the warmest periods in earth history after the dinosaur-killing asteroid impact around 70 million years ago (including the Paleocene-Eocene Thermal Maximum [PETM] about 55 million years ago), with a subsequent gradual cooling for 40 million years.
- Antarctica began to accumulate its ice sheet about 33 million years ago
- Pliocene into Pleistocene (5 to 1 million years ago)
- Continued cooling, with Northern Hemisphere ice sheets forming about 2.6 million years ago, with periodic advances and retreats caused by eccentricities in the earth’s orbit.
- Latter Pleistocene to last glacial maximum (LGM, 1 million to 20,000 years ago)
- Still marked by advances and retreats of the land ice sheets
- LGM to present day
- Emergence from the LGM, with pauses resulting from disruption of the ocean circulation about 9,000 years ago, which lasted about 750 years.
Proxy quality
For more recent climate/temperature, ice and existing lake and ocean sediments are usually more than adequate. Greenland ice core data goes back about 100,000 years, while Antarctica data goes back even further, about 750,000 years. Unfortunately, the further back we go in time with these proxies, natural processes “smear” out the details, such as compression weight creating sedimentary rocks and ice sheets. Even processes down to microbial scales may distort the time and proxy values of a paleoclimatological sample. However, recent advances in evaluating sample proxy values and their age have made it possible to get climate information at thousand-year resolution as far back as beyond the PETM. This work has given some interesting results that I’ll talk about later.
What Climate Models Tell Us At Present
the means
Future forecasts are corrected for biases they may have, by comparing observations over a recent period to climate simulations over that period. The most recent projections, from IPCC AR6, are shown below. The assumptions for CO2 emissions are detailed in the IPCC AR6 report on pp SPM-15 through SPM-16, and briefly summarized in the caption. Unfortunately, the results are all too familiar.
the extremes
In this new report, estimates are made of the frequency of what was a one in ten and one in 50 year event in the last half of the 19th century (before there was extensive human influence on the climate), in scenarios with 1o (current), 1.5o, 2o and 4oC warming.
This diagram represents the global average increase in frequency for these temperature extremes over land, rather than a specific value for a region. Drought moves from a current value 1.7 times greater than the base period, with the current increase of 1oC, to 4.1 times greater for a 4oC mean global temperature rise. One in ten year heavy rain events move from 1.3 times more frequent today to 2.7 times as frequent with a 4oC mean global temperature rise.
what don’t we know: a new wrinkle from pETM proxy data?
A question remains to be answered: Are there high frequency extreme events that have occurred in the past, and what were they caused by? An article in Science Advances examines data using stable isotopes of carbon (C-13) and oxygen (O-18) to find out the nature of rapid changes in temperature from 54 to 46 million years ago. It turns out there’s a strong tendency toward amplified extreme events, with global mean temperature increases of as much as 5-8oC. This behavior is thought to be an essential part of their nature. However, these amplified warm events stopped occurring when ice sheets became part of the earth climate system about 33 million years ago.
While the time resolution of the data studied is only on the order of thousands-of-years, the result may be relevant to the larger-than-expected precipitation and temperature extremes occurring today. The PETM extremes are thought to be related to rapid changes in CO2 and methane concentrations in the atmosphere, which may mean that the climate system is more sensitive to CO2 and methane than previously thought. Whether that’s true on the decadal to century time scales can’t be determined at this point.
Conclusions
We already have seen more extreme heat, precipitation, and drought in the last couple of years than was expected by most climate scientists. The record from earlier warm periods can now be examined at a resolution of 1,000 or so years. Looking at a period much warmer than today, large sudden warming events on the order of a couple of thousand years could be seen, which would make extreme events more likely. This would also mean that the climate system may be more sensitive to greenhouse gas concentrations than previously thought.
The establishment of ice sheets (and sea ice) seems to have damped out the extreme and rapid warming events from 33 million years ago to the present. It couldn’t be determined what critical value of ice loss is needed for rapid development of extreme warm events to return.
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