2013 was the first time planet Earth passed 400ppm carbon dioxide in the atmosphere since the Pliocene perod. Instruments at the Mauna Loa Observatory in Hawaii have been measuring atmospheric CO2 and other greenhouse gases for over 50 years. Other sites like CSIRO's Cape Grim observatory in Tasmania and New Zealand's National Institute of Water and Atmospheric Research (NIWA) at Baring Head Station near Wellington also take baseline measurements of atmospheric greenhouse gases.
The Keeling curve shows the rising levels of atmospheric carbon dioxide trend and is one of the most well known climate graphs. It was named after Charles Keeling from the Scripps Institute for Oceanography who started measuring CO2 at Mauna Loa in 1958 and continued up to his death in 2005. The graph clearly shows the northern hemisphere season cycle of carbon dioxide as part of the biological seasonal processes as CO2 is taken up by vegetation in Spring and Summer, then expired in Autumn and Winter, but the long term trend has been a gradual rising of CO2 by about 2ppm per year.
The significance of reaching 400ppm is that the planet has not seen this level of carbon dioxide for 3 to 5 million years, in the Pliocene Period. It was a warmer world then. By examining this geological period we can gain some insight on what we might expect to happen to temperatures, sea levels, the impact on ice sheets, and some signs of how our climate might change later this century and further into the future.
In this graph of the Keeling curve up close with just a couple of years data you can clearly see the seasonal nature of atmospheric CO2 and still discern the long term upward trend Source: keelingcurve.ucsd.edu
Arctic had a warmer more equable climate with no sea ice
The Pliocene was marked by warmer and cooler periods following the changes in orbital obliquity of the Milankovitch 40,000 year cycles, but with an overall cooling trend. The peak carbon dioxide levels of 400ppm and temperatures of 2 to 3.5C higher than today were reached in the 300,000 year long Mid-Pliocene Warm Period (MPWP)
(Raymo et al 1996) put forward that global temperatures may have been up to 3.5C warmer than the present with CO2 about 35% higher than the pre-industrial value of 280ppm. In their paper they present carbon isotopic evidence for a stronger thermohaline circulation in the Atlantic Ocean during the warmest intervals of the Pliocene. They put forward that this stronger North Atlantic current may have acted as a positive feedback enhancing sea ice retreat and reducing the Arctic albedo .
More recent research has shown that during the middle Pliocene the Arctic was much warmer and ice free (Scientific American). The previous scientific orthodoxy was that the Arctic sea ice extent went up and down but was substantially less than what we see today.
Modelling experiments by (Ballantyne et al 2013) to match terrestrial surface temperatures showed that the absence of Arctic sea ice provided the best simulation for land temperatures measured via proxies. This study also showed that "Continental interiors were warmed by water vapor advected from the ice free ocean" to create a more equable Arctic climate.
According to (Ballantyne et al 2010) in a study titled Significantly warmer Arctic surface temperatures during the Pliocene indicated by multiple independent proxies said that:
"Arctic temperatures were ∼19 °C warmer during the Pliocene than at present, while atmospheric CO2 concentrations were ∼390 ppmv. These elevated Arctic Pliocene temperatures result in a greatly reduced and asymmetrical latitudinal temperature gradient that is probably the result of increased poleward heat transport and decreased albedo. These results indicate that Arctic temperatures may be exceedingly sensitive to anthropogenic CO2 emissions."
Other research confirms the lack of Arctic sea ice. Deep sediment cores from the Bering Sea revealed that the area was ice free all year with high biological productivity, according to Christina Ravelo, Professor of ocean sciences at the University of California, Santa Cruz. Sea surface temperatures were about 14 degrees, at least 5 degrees warmer than today.
Research by a team of US and Russian scientists lead by Professor Julie Brigham Grette analysed Lake bed sediments in Siberia which go back as far as the Mid Pliocene Warm Period. The study (Julie Brigham Grette et al 2013) examined Pliocene Warmth, Polar Amplification, and Stepped Pleistocene Cooling Recorded in NE Arctic Russia:
Evidence from Lake El'gygytgyn, in northeast (NE) Arctic Russia, shows that 3.6 to 3.4 million years ago, summer temperatures were ~8°C warmer than today, when the partial pressure of CO2 was ~400 parts per million. Multiproxy evidence suggests extreme warmth and polar amplification during the middle Pliocene, sudden stepped cooling events during the Pliocene-Pleistocene transition, and warmer than present Arctic summers until ~2.2 million years ago, after the onset of Northern Hemispheric glaciation. Our data are consistent with sea-level records and other proxies indicating that Arctic cooling was insufficient to support large-scale ice sheets until the early Pleistocene.
The 24 minute youtube video of Julie Brigham Grette (below) mainly focusses on the core results from the Pleistocene, but it provides an excellent introduction into the research undertaken with implications for the later paper on the results on the Pliocene period.
The Brigham Grette study provides evidence from pollens embedded in sediment that Conifer forests also grew where tundra now stands in Siberia. It is likely these forests extended across Alaska and Canada and even to northern Greenland providing evidence of a warm and stable Arctic climate.
The study also reveals substantial correlation between Arctic and Antarctic warm periods. See my article on Ice Sheets and Sea level: what the past tells us is likely from December 2013.
Warm tropical sea surface temperatures
Of course working out the feedback mechanisms in play in the Pliocene is not a simple process to explain the gradual cooling trend through the entire period. (Federov et al 2013) in Patterns and mechanisms of early Pliocene warmth looks at geochemical proxy records of sea surface temperatures that show the early Pliocene climate had "substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures."
They argue that none of the mechanisms put forth so far adequately explain these three features and suggest that a number of dynamical feedback mechanisms like ocean mixing and cloud albedo may be underestimated in present models.
In this youtube video Chris Brierley, a climate modeller, discusses the (Federov et al 2013) Nature paper which he was a co-auther of - Patterns and mechanisms of early Pliocene warmth (abstract) The study analysed sea surface temperatures with Brierly suggesting that changes in Pliocene rainfall in a warm world over East Africa may have been an important factor driving human evolution.
The study highlights that there was a giant mass of warm water in the tropics that stretched out from Indonesia over to Africa and South America about 4 million years ago.
“Essentially, we’ve looked at a warm world in the past and it shows changes in the pattern of tropical sea surface temperatures. We’ve analyzed all the existing theories to explain this vast pool of ancient warm water, and even in combination they can’t explain something as odd as this,” said co-author Dr Chris Brierley of University College London as reported in this Sci-news article from April 2013.
Permanent El Nino state in the Pacific?
During much of the Pliocene the pacific may have developed "a permanent El Niño-like state" according to (Alan M. Haywood et al 2007) which may have been a positive feedback to the warming climate perhaps increasing annual global mean surface temperatures by up to 0.6°C of warming. Minor, but not insignificant. This would have changed rainfall patterns and the level of cloud and cloud albedo affecting the climate.
Recent research is already indicating that in the present day El Niño Southern Oscillation activity and intensity increasing with Global Warming. (McGregor, S. et al 2013)
Global surface air temperature anomalies in the Pliocene four million years ago (A. V. Fedorov et al 2013) Still image sourced from Youtube Video.
Sea levels were 20 to 30 metres higher indicating reduced Ice sheet mass
For most of the Pliocene period sea levels were about 20 meters higher or more than today, which would account for loss of the Greenland and West Antarctic Ice sheets and a component from the East Antarctic Ice sheet. (DeConto and Pollard 2013)
(Naish et al 2009) argue that the earth's obliquity cycle with a 40,000 year periodicity played a major role in regulating the climate of the Pliocene period. In a seperate paper by (Naish and Wilson 2009) they argue that the oxygen isotope ratio in foraminafera record from ocean sediments "imply orbitally influenced sea-level fluctuations of up to 30 m amplitude during the Mid-Pliocene, and up to 30 per cent loss of the present-day mass of the East Antarctic Ice Sheet (EAIS) assuming complete deglaciation of the West Antarctic Ice Sheet (WAIS) and Greenland."
With Arctic ice completely gone and Antarctic ice much reduced, the planetary albedo would be much smaller allowing the earth to absorb more solar radiation and heat up. Elevated carbon dioxide levels would increase greenhouse warming as a positive feedback. Increased water vapour in the atmosphere would have also been a positive feedback mechanism.
The biological pump would have been more active in the carbon cycle as a negative feedback. With warming temperatures, the water cycle would be more active increasing weathering as a long term negative feedback. Ocean mixing and cloud albedo may have also played a role as feedback mechanisms.
The closure of the Panama Isthmus towards the end of the Pliocene at around 3 million years ago is likely to have intensified the thermohaline circulation in the North Atlantic according to modelling by (Lunt et al 2007), although modelling reveals it would have played an insignificant part in the increased cooling and glaciation happening in the Arctic at the end of the Pliocene.
(Lunt et al 2008) argues that the main reason for onset of the Greenland Ice sheet about 3 million years ago was a decrease in atmospheric carbon dioxide, rather than the closure of the Panama seaway.
By the end of the Pliocene CO2 levels were down around 200ppm, with global temperatures also falling and climate deteriorating rapidly. The Greenland ice sheet established itself and sea ice and glaciation had returned to set in place the Ice Ages of the Pleistocene Period.
CO2 increasing much more rapidly than in the Pliocene
One thing to remember is that the changes in CO2, temperature and ice extent ocurred over thousands and tens of thousands of years in the Pliocene enabling species to adapt to the changing climate. Our industrial and agricultural processes are increasing carbon dioxide much more rapidly than occurred naturally in the Pliocene, at least 10 times or more as strong. This may cause abrupt adjustments in our climate system, according to Eelco Rohling, Professor of Ocean and Climate Change at Southampton University. He highlighted the possibility of abrupt climate change during a presentation at American Geophysical Union Fall meeting 2011 in San Fransisco where he compared climate being like a huge elastic band which we are stretching. Look out for the snap!
"We have this relationship, what is the basic underlying natural relationship between sea level and climate forcing and if we use that, then at 1.6 watts per square metre forcing where we are at the moment, then we would expect the equilibrium sea level in the natural state to be 25 metres (±3-5 metres) above the present. So this is essentially the elastic band of climate that we are stretching. We are creating a disequilibrium. Now the problem is we are not only stretching but we're stretching it really quickly so the system cannot keep up at all. ...The adjustment is similar to yanking an elastic band really quickly - there could be snaps in there - and we could have very abrupt adjustments happening.
This article is a little bit of a tangent to my regular articles. I am presently doing an online course - Climate Change: Challenges and solutions - offered by the University of Exeter (UK). So please indulge me as I also use this blog for some climate course work. This article is for section 2.6 of the course on discussing passing the 400ppm CO2 in the atmosphere and comparing it to the Pliocene period when we last had a similar concentration of atmospheric CO2.
- Robert DeConto and David Pollard, Geophysical Research Abstracts, Vol. 15, EGU2013-13279-2, 2013 - A smaller Antarctic Ice Sheet in the Pliocene and in the Future (abstract)
- Naish et al, 2009 Nature - Obliquity-paced Pliocene West Antarctic ice sheet
oscillations (abstract) (Full Paper PDF)
- Tim R Naish and Gary S Wilson, Phil. Trans. R. Soc. A 13 January 2009 vol. 367 no. 1886 169-187, Constraints on the amplitude of Mid-Pliocene (3.6-2.4 Ma) eustatic sea-level fluctuations from the New Zealand shallow-marine sediment record (Full Paper) Published 13 January 2009 doi: 10.1098/rsta.2008.0223
- Raymo M.E, Grant B, Horowitz M, Rau G.H 1996 Mid-Pliocene warmth: stronger greenhouse and stronger conveyor. Mar. Micropaleontol. 27, 313-326. doi:10.1016/0377-8398(95)00048-8 (abstract)
- Daniel J. Lunt Paul J. Valdes Alan Haywood Ian C. Rutt, Climate Dynamics (2008) 30:1-18 DOI 10.1007/s00382-007-0265-6 Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation (abstract) (PDF)
- Daniel J Lunt et al, 2008, Nature, Nature 454, 1102-1105 (28 August 2008) | doi:10.1038/nature07223; Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels (abstract)
- Carys P Cook et al, 2013, Nature Geoscience 6, 765-769 (2013) doi:10.1038/ngeo1889 Dynamic behaviour of the East Antarctic ice sheet during Pliocene warmth (abstract
- A. V. Fedorov, C. M. Brierley, K. T. Lawrence, Z. Liu, P. S. Dekens & A. C. Ravelo, (2013) - Patterns and mechanisms of early Pliocene warmth Nature 496, 43-49 (04 April 2013) doi:10.1038/nature12003 (abstract)
- A.P. Ballantyne,D.R. Greenwood,J.S. Sinninghe Damsté, A.Z. Csank, J.J. Eberle and N. Rybczynski - Significantly warmer Arctic surface temperatures during the Pliocene indicated by multiple independent proxies (Full Paper) (2010), Geology, Geological Society of America, doi: 10.1130/G30815.1, v. 38 no. 7 p. 603-606
- Ashley P. Ballantyne, Yarrow Axford, Gifford H. Miller, Bette L. Otto-Bliesner, Nan Rosenbloom, James W.C. White. The amplification of Arctic terrestrial surface temperatures by reduced sea-ice extent during the Pliocene (abstract). Palaeogeography, Palaeoclimatology, Palaeoecology, 2013; DOI: 10.1016/j.palaeo.2013.05.002
- Alan M. Haywood, Paul J. Valdes, Victoria L. Peck, Paleoceanography (2007), DOI: 10.1029/2006PA001323 - A permanent El Niño-like state during the Pliocene?, (abstract)