Back to the Future: Do Ancient Climates Predict Our Climate Future?
The Pliocene and Eocene as analogs for near-future climates
Sea level rise may be the most damaging outcome of a warming climate system in the long run, and the most difficult to avoid or alter. The Earth has not seen temperatures like those predicted for the end of this century for more than 3 million or maybe 30-50 million years. This essay asks: What did the Earth look like in those ancient geologic eras?
Global Temperature and Sea Level Over Geologic Time
Sometimes a single image can change the way you see the world. That happened when I came across a paper with Dr. Kevin Burke as lead author offering a perspective on climate change that I had not considered before. The concept was that we can look to past eras in the Earth’s history as analogs for our climate futures.Â
A picture is truly worth (much more than) a thousand words, and this one from the Burke paper is a brilliant presentation summarizing a telescoping view of changes in global temperature both recently and over geologic time.
Note the change in scale along the time axis, from years on the far right, through thousands of years (kyr) in the middle, and then millions of years (Myr) at the far left. This one graph captures the current rapid increase in temperature projected out to 2300, the dynamics and range of temperatures during the ice ages (with a 100,000 year cycle – middle panel) and then looks farther back in time.
While this history and its causes are fascinating stories on their own, the authors also use this exceptionally informative graph to pinpoint two periods in this long story that might offer analogs and insights into the climate future that awaits us.Â
A simple summary of their in-depth analysis comes from comparing the far right panel with the longer-term changes in temperature to the left. They propose that the lower projections of how much we will warm the planet by 2100 (~ 2 degrees Celsius) would bring the average global temperature to that of the middle Pliocene (~3 million years ago). More extreme estimates (~5 degree increase) would bring that global average back to that of the late Eocene (~30-50 million years ago).
Dr. Burke and colleagues talk of winding the climate clock back 3 million and 30-50 million years to catch a glimpse of our future. Back to the Future indeed. There will be some caveats on the use of these analogs at the end of this essay, but let’s not break the narrative train just yet. Instead, lets ask:
What did the Earth look like in those eras?Â
For simplicity, we’ll look at just three major characteristics: Temperature, vegetation types, and average sea level.
The Pliocene
Climate models predict that ocean temperatures in the Pliocene (3 million years ago) were up to 8 degrees Celsius higher than today, with greater warming at the poles and little at the equator. One exception is the predicted warming off the coast of Peru where the El Niño/La Niña (or ENSO) oscillation occurs, with significant impacts of weather in many parts of the world. Some see a return to a more frequent if not permanent El Niño-like condition as part of our climate change future.Â
Predicted vegetation types for the Pliocene show some significant differences from what we see today including grasslands across much of the currently-forested Eastern U.S., and deciduous instead of evergreen (boreal) forests across Canada and Scandinavia.
The bigger difference is in sea level which can rise due to thermal expansion (water expands as it warms) or by addition of new water through melting of land-based ice (glaciers and ice caps). Current rates of sea level rise are about one-third due to expansion and two-thirds to addition.Â
In this modeled Pliocene world, permanent ice fields in Greenland were restricted to the northeastern quadrant of that large island, and sea levels were ~15 meters (~50 feet) higher than current! Impacts already ascribed to rising sea levels in our warming world have occurred with a rise of about 10 inches (less than one-third of a meter). Sea level has fluctuated by as much as 100 meters in the last 800,000 years as the massive glaciers we now know were associated with the ice ages waxed and waned, emphasizing that the amount of land-based ice in the world is a dominant factor in determining sea level.
The Eocene
The Eocene scenario is even more extreme, but the ground rules have changed a bit as well. Three million years is not enough time for significant movement of the continents, but fifty million years is.  At that time, the Indian subcontinent had not yet plowed north into Asia, causing the rise of the Himalayan mountains, Australia was moving northward into the climatic desert zone centered on 30 degrees south of the equator, and permanent ice fields were absent in both Greenland and Antarctica.
Despite these challenges, another excellent recent paper with Dr. Nicholas Herold as first author has summarized the combination of land masses in motion and the absence of ice in this view of the Earth at that time.
Models of climate zone distributions for this very different time suggest that tropical forests covered much of North America while evergreen conifer forests of the current boreal zone were absent. There was more dominance of dryland vegetation types, and most significantly, all permanent ice fields were gone; Antarctica supported both tundra and forested systems!Â
All of which would predict much higher sea levels. Describing those given the differences in the location and topography of continents is a little tricky, but estimates range from 60 to 70 meters (~200 feet) above current levels.
Current estimates of sea level rise that would occur if all of Greenland and Antarctic ice melted are also around 70 meters. Those two massive accumulations of ice contain ~99% of the ice and 68% of the fresh water on Earth.Â
Caveats
The Pliocene and Eocene examples, driven by estimated changes in global temperature of 2 and 5 degrees Celsius, project stunningly different worlds than we now inhabit. Temperatures in the far northern hemisphere would be much higher than the current global average. Vegetation changes would be extreme with major biomes shifting towards the poles or disappearing altogether. Imagine an Antarctic continent devoid of ice and covered in grasses, trees and shrubs.
But the elephant in the room is sea level rise. All of the current concerns and outcomes related to sea level rise in our warming world have resulted from cumulative increases of about a third of a meter. These epochal Pliocene and Eocene scenarios suggest rises of from 15 to 70 meters, or 45 to 210 times what we have experienced to date.
The major caveat then for the relevance of these scenarios to our climate future is one of time scale. These scenarios represent Earths that have had millions of years to respond to changing temperatures. They are at equilibrium, if you like, in terms of ice and vegetation coverage. A fundamental truth about our current climate dynamic is that we are NOT in equilibrium. We are forcing change at a rate that is unprecedented in the geologic record.Â
Two major reservoirs in the Earth’s climate system are resisting our movement towards the Pliocene and Eocene worlds: Oceans and Ice.Â
The global ocean circulation system has, to date, absorbed about 90% of the excess heat produced by greenhouse gas increases, delaying global temperature increases significantly. This circuit, also called the Thermohaline system, takes about 1,000 years to complete a turn. Heat energy transferred to the oceans will resurface eventually, and impact atmospheric temperatures, but on a time-scale of hundreds to thousands of years.
And while the rate of ice melt is increasing in both Greenland and Antarctica, current projections call for sea levels to increase by less than 1 meter through 2100; far less than 15-70 meters!  The Antarctic ice sheet is as much as 3,000 to 4,000 meters thick and at current rates of loss will take more than 100,000 years to disappear completely. There is some uncertainty about future loss rates.
So while we are moving toward a Pliocene/Eocene future as described by global temperatures, inertia in both the marine and ice systems of the Earth will delay that new equilibrium in terms of sea level rise for millennia.Â
This is good news.
Still, maybe it is time to start extending the timeline of our climate projections well beyond 2100 (the first figure at the top of this essay has done that). Children born today have a good chance of seeing 2100, and their children and grandchildren might see 2150 and beyond.
The Pliocene and Eocene are out there as signposts pointing the direction in which we are headed. Maybe with this perspective we can think about how to slow our progress back into the future as we prepare for now inevitable increases in sea level.
Sources
The paper by Burke et al, including the first figure in the essay, is here:
https://www.pnas.org/content/115/52/13288
A description of Pliocene climate is here:Â Â Â Â Â
https://commons.wikimedia.org/wiki/File:Pliocene_sst_anomaly.png
https://en.wikipedia.org/wiki/Pliocene_climate
The map of Pliocene veg is modified from: https://upload.wikimedia.org/wikipedia/commons/9/9e/Pliocene_megabiome.png
https://en.wikipedia.org/wiki/Pliocene_climate
Projected distributions of forest species and forest types for the U.S. are derived by the Forest Inventory and Analysis program of the U.S. Forest service and can be found here:
https://www.nrs.fs.fed.us/atlas/tree/future_iv_318.html
Pliocene sea level rise estimate is from:
https://royalsocietypublishing.org/doi/10.1098/rsta.2012.0294
Measurements of modern sea level rise (since 1900) are presented here:
https://climate.nasa.gov/vital-signs/sea-level/
The Eocene paper with Herold as first author, and including both the map of the Earth’s surface and the distribution of vegetation types, is here:
A reference for consistent distribution of temperatures now and in the Eocene is here:
https://royalsocietypublishing.org/doi/10.1098/rsta.2013.0123
and some graphics are here:
References for a range of sea level rise in the Eocene include:
https://en.wikipedia.org/wiki/Past_sea_level
https://commons.wikimedia.org/wiki/File:Phanerozoic_Sea_Level.png
https://royalsocietypublishing.org/doi/10.1098/rsta.2012.0294
Estimates that 99% of fresh water on Earth is in the Greenland and Antarctic ice sheets, and that sea levels would rise 66 meters if both melted completely are from:
https://nsidc.org/cryosphere/quickfacts/icesheets.html
A reference for 90% of excess heat energy due to greenhouse gases having been absorbed by the ocean is here:
https://www.ncei.noaa.gov/news/ocean-heat-content-rises
A description and depiction of the Thermohaline circulation system is here:
https://en.wikipedia.org/wiki/Thermohaline_circulation
Estimates of how long it might take for all Antarctic ice to melt are calculated from data found here:
https://en.wikipedia.org/wiki/Antarctic_ice_sheet
The projection for actual sea level rise through 2100 is from the IPCC Sixth Assessment report:
https://www.ipcc.ch/assessment-report/ar6/
The image of the global thermohaline circulation system is from: