On Monday this week, and rather late in the day, Dr Colin Summerhayes from the Scott Polar Institute, Cambridge University, left this lengthy comment at the end of the thread on Prof. Richard Lindzen’s post called Global Warming and the Irrelevance of Science. I wanted to respond to some of the points raised but did not want to do this at the end of an old comment thread, and so asked if I may publish the comment as a guest submission. Dr Summerhayes responded by expanding on the comment and submitting the Opinion Piece that is published below.
Here are some of the highlights from Dr Summerhayes’ CV:
- April 2010: Emeritus Associate, Scott Polar Research Institute
- January 1 2004 part time, and full time from April 1 2004- April 9 2010: Executive Director, International Council for Science’s Scientific Committee on Antarctic Research (SCAR)
- 1997-2004: Director Global Ocean Observing System (GOOS) Project Office; UNESCO’s Intergovernmental Oceanographic Commission, Paris
- 1995-1997: Southampton Oceanography Centre; Deputy Director, and Head of Seafloor Processes Division.
- 1988-1995: Director, Natural Environmental Research Council’s Institute of Oceanographic Sciences Deacon Laboratory, Wormley, Surrey.
- 1982-1988: BP Research Centre. (A) 1982-1985: Research Associate; (B) 1985-88: Senior Research Associate and Manager, Stratigraphy Branch.
- 1976-1982: Research Associate and Project Leader; Petroleum Geochemistry Branch, Exxon Production Research Co, Houston, Texas.
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Despite the world’s politicians finally agreeing, in Paris in December 2015, on what to do about global warming, many scientists still reject the evidence for it being caused by humans, or question that it is a significant problem.
For example, Dr Lindzen (2016) agrees that although carbon dioxide (CO2) is a greenhouse gas, which absorbs and re-emits infrared radiation from the Earth’s surface, the increase in its concentration in the atmosphere is not important because its climate sensitivity (the amount by which temperature will rise for a doubling of CO2) is low. This common rejection of the anthropogenic global warming theory ignores the evidence from the Earth science community about the causes of climate change in the past (for summaries see e.g. Ruddiman, 2013; Hay 2013; Summerhayes, 2015). The chapter on Palaeoclimate in the report on the science of climate change by Working Group 1 (WG1) of the Intergovernmental Panel on Climate Change (IPCC) summarizes some of this evidence (Masson-Delmottee et al, 2013), but almost nobody reads it, and the media largely ignores it. If we want to know what causes climate change at any one time we have to divide the climate signal into its constituent parts and look at the forcing provided by each element.
John Tyndall, the experimentalist who discovered that carbon dioxide, methane and water vapour act as what we now call greenhouse gases (Tyndall, 1861), considered that his discovery opened the way to understanding the wanderings of Earth’s climate uncovered by geologists. One of the first geologists to take him seriously was a Swede, Arvid Hogböm, who asked Svante Arrhenius if he could calculate whether fluctuations in atmospheric CO2 might account for glacial to interglacial changes of the Ice Age (Arrhenius, 1896). Yes, said Arrhenius, who later coined the term ‘Hothouse [now Greenhouse] Climate’ (Arrhenius, 1908). Geologist T.C. Chamberlin converted Arrhenius’s findings into a geologically based conceptual climate model (Chamberlin, 1897, 1899). Volcanoes emitted CO2, which was extracted from the air by chemical weathering. Imbalances over time between the volcanic source and the chemical weathering sink accounted for periods of warming or cooling.
For a full accounting of the effects of CO2 we had to wait until its spectrum had been established by the investment of the US military in large spectrophotometers in the early 1950s. Physicist Gilbert Plass used the new data to confirm that fluctuations in CO2 had likely contributed to the variability of the climate of the Ice Age (Plass, 1956). Roger Revelle was convinced it was important to measure CO2, and set about finding funds for Charles Keeling to set up a CO2 measuring facility on Mauna Loa in 1957 (Revelle and Suess, 1957). Keeling ignored CO2 measurements from sites close to industrial plants or cities where CO2 was being emitted. He wanted background air (Keeling, 1960).
Charles Lyell was among the first to note that Earth had experienced a ‘Great Cooling’ since Cretaceous times (Lyell, 1830), and it was soon realized that there had been a previous glaciation in Carboniferous-Permian times (Ramsay, 1855). During Lyell’s Great Cooling, plate tectonic processes were moving the continents apart from one another, eventually isolating Antarctica at the South Pole 90 million years ago.
In the late 20th century, geochemist Bob Berner (Berner, 1999, 2004), and palynologists like Dana Royer and Dave Beerling (Beerling and Royer, 2011), confirmed that the Great Cooling was likely driven, as T.C. Chamberlin had suggested, by changing atmospheric CO2 through time (Royer et al., 2004). Plate tectonics processes provide abundant CO2 from volcanic activity (the source) associated with phases of rapid seafloor spreading, and soak up CO2 during phases of mountain building that enhance chemical weathering (the sink) (Berner, 1999). We now have various ways of estimating the likely abundance of CO2 in the air in past times (e.g. see Beerling and Royer, 2011). This modern understanding is remarkably similar to T C Chamberlin’s conceptual model.
Chemical weathering works by atmospheric CO2 combining with H2O vapour to form a weak acid that attacks silicate and carbonate minerals exposed by mechanical weathering in mountains, and the silicates of flood basalts. It absorbs CO2 from the atmosphere (Kent and Muttoni, 2013). The products of chemical weathering, including that CO2, are taken to the sea by rivers, consumed by plankton, and used to build soft organic tissue and calcium carbonate skeletons (Berner, 1999). Much of this is recycled in the water column when organisms die, but some falls to the seafloor to be preserved as sediment (another CO2 sink).
About half of the CO2 put into the air dissolves in the ocean to enable these two ‘fluids’ to stay in chemical equilibrium. Dissolution of CO2 in the ocean makes it slightly more acid, which makes deep bottom waters corrosive to sediments rich in CaCO3 skeletal remains. The boundary below which carbonate cannot accumulate is termed the carbonate compensation depth (CCD), and is a marker for the amount of CO2 dissolved in the ocean. When CO2 is abundant the CCD is shallow, as it was in Eocene times some 50 million years ago (Pälike et al., 2012).
When rates of seafloor spreading are high (hence CO2 is abundant and the climate is warm), the ocean becomes depleted in magnesium (Mg), which favours deposition of low-Mg-Calcite. When rates of seafloor spreading are low (hence CO2 is less abundant and the climate is cool), the ocean becomes enriched in Mg, which favours the deposition of Aragonite (Müller et al, 2013). The oscillation between these two types of calcium carbonate (from Calcite to Aragonite) is known as the calcite metronome (Zalasiewicz And Williams, 2012), and is an indirect reflection of (though not caused by) the abundance of CO2 in the atmosphere.
Deep sea cores contain records of a massive emission of carbon at the Palaeocene-Eocene boundary, 55 million years ago. It caused a rise in temperature of 5-6°C, and acidified the deep ocean, raising the CCD by about 2 km and killing off deeper water organisms with carbonate skeletons (Zachos et al., 2010). Sea level rose by 10-12 m. New work shows that this emission occurred in two very closely spaced events in which the rise in CO2 was almost as abrupt as it is today (Bowen et al, 2015), making this a possible natural analog for what we are now doing to the climate.
Deep sea cores also show that around 34 million years ago the production of cold Antarctic Bottom Water began (Kennett 1977, Zachos et al., 2001). That in turn suggested that gaps had opened between Antarctica and Tasmania and Tierra del Fuego, permitting the formation of the Antarctic Circumpolar Current (ACC) driven by westerly winds, and thermally isolating the continent (Kennett 1977). It has also been suggested that the gradual decline in atmospheric CO2 cooled the continent leading to the growth of ice caps and then, 34 million years ago, to the formation of a major ice sheet (DeConto and Pollard, 2003; Pollard and DeConto, 2005). Supporting evidence for the CO2 hypothesis comes from Southern Ocean sediments containing debris from Antarctic glaciers in the Eocene, before the ACC developed (Barrett, 1999).
There is also evidence, from bubbles of fossil air in ice cores, that rising temperature increases atmospheric CO2. When temperature rose, so did the CO2, which emerged from the ocean as it warmed, as expected from the operation of the Gas Laws. That rise in CO2 warmed temperatures further through positive feedback. When the CO2 signal was found in the Vostok ice core, it appeared that the CO2 signal lagged the temperature signal (Petit et al., 1999). But, in 2013, Frederic Parrenin showed that during the last glacial termination CO2 changed with temperature (Parrenin et al., 2013). The apparent delay at Vostok now seems to be an illusion generated by the age model for the air in ice core bubbles.
During the Ice Age of the past 2.6 million years our climate was predominantly cool, interrupted by brief warm intervals (interglacials), like the one we are now living in. This pattern is caused by regular periodic changes in the Earth’s orbit and the tilt of the Earth’s axis, which change the amount of incoming solar radiation (insolation). The changes in that insolation are not large enough to account for the amount of warmth in interglacials. The extra warmth most likely comes from the release of CO2 from the oceans as they warm (Petit et al, 1999).
In the last interglacial, 120,000 years ago, temperatures were 2-3°C warmer than today, and sea level rose 4 to 9 metres above today’s level (Kopp et al., 2009). Geology is telling us that if we raise our temperature that much, our coasts will be inundated, as they were then. We should be concerned about that.
Insolation peaked around 11,700 years ago, melting the great North American and European ice sheets and parts of West and East Antarctica. Since then, insolation has declined (Berger and Loutre, 2002). As a result, Earth’s climate cooled over the past 10,000 years (Marcott et al., 2013; see also the PAGES 2k Consortium, 2013, for the past 2000 years). The cooling trend culminated in the Little Ice Age of 1350-1850.
Orbital calculations show that we should remain in this cold condition for the next 5,000 years (Berger and Loutre, 2002). So, why are we not still in the Little Ice Age? Has the sun suddenly gotten hotter? No. The latest data on sunspots (Clette et al, 2014) show that there were about the same peak numbers of sunspots in the 1980s-90s as there were in the 1780s or the 1840s-60s, during the Little Ice Age. Indeed, the number of sunspots has been declining since 1990 (Clette et al, 2014; Lockwood, 2010), so our climate should be cooling from that cause too. Why is it not?
What reversed the cooling trend of the past 10,000 years, if it was not the Sun? The finger points clearly at the rise in our emissions of CO2 and related greenhouse gases (especially methane and nitrous oxide). Data from ice cores show that the abundance of CO2 in the atmosphere increased from 280 ppm to 400 ppm, starting in about 1770 (Wolff, 2011). That’s when James Watts’ steam engines started burning coal in abundance, kicking off the industrial revolution. The rise in CO2 was exponential, starting slow, taking off as we entered the 20th century, and increasing even faster after about 1955. Not surprisingly, given what we know from geology and from basic physics, temperature increased along with it. Proof that the source is the burning of fossil fuel comes from carbon isotopes, which show an increase in the concentration of the 12C as opposed to the 13C carbon isotope.
Couldn’t the present warming be something like that which caused the Medieval Warm Period (MWP)? We can answer that question with recourse to the record of beryllium-10 (10Be) and carbon-14 (14C) isotopes in (e.g.) tree rings (Steinhilber et al., 2012). Cosmic rays create these isotopes in the upper atmosphere. The two isotopes were abundant when the Sun’s output was weak, and the climate was cool, and rare when the Sun’s output was strong, and the climate was warm. The Sun’s output follows the 208-year Suess Cycle and the 88-year Gleissberg Cycle. The isotopic patterns show that these cycles should have made the climate warm during the MWP, but not as warm as the mid 20th century (Steinhilber et al., 2012). Equally, these patterns show that the Little Ice Age was not uniformly cold, but characterized by a series of cold peaks (solar minima), separated by warm periods. The last such cold peak was the Gleissberg Minimum in about 1900, after which the Sun’s activity rose to a broad peak between 1960 and 1990, followed by a decline (Lovelock, 2010; Clette et al., 2014). These solar effects are smaller in magnitude than those caused by orbital insolation. So the curve of declining temperature with time over the past 2,000 years is driven primarily by orbital change, superimposed on which are small warmings (like the MWP), and coolings (like those of the solar minima of the Little Ice Age) (Summerhayes, 2015). The current warming is now back at about the level of the Roman Warm period. It is most extreme in the Arctic, where plants that last saw daylight several thousand years ago are emerging from beneath Canadian Ice Caps. The modern warmth of the eastern Canadian Arctic now exceeds the peak warmth of the early Holocene, when summer insolation across the region was 9% greater than present (Miller et al., 2013).
Why is there not a perfect 1:1 relationship between the subsequent rises in temperature and CO2? Small periodic natural fluctuations in the climate system, like warm El Niño events followed by cold La Niña events, temporarily disrupt the basic temperature-CO2 link. Another such fluctuation is the Pacific Decadal Oscillation, which warms the Pacific slightly for 20-25 years, then cools it for as long (Chavez et al., 2003). Since about the year 2000 we have been in the negative (cool) phase of the PDO. It has operated like a prolonged La Niña event, keeping Earth slightly cooler than it would otherwise have been (Fyfe et al., 2016). In a few years from now the PDO will revert to its positive (warm) phase, and operate like a prolonged El Niño, warming the planet once more. These relatively small natural fluctuations are superimposed on the underlying trend driven by the CO2-temperature relationship that, along with orbital change, has modulated our climate for millions of years. We are now perturbing it.
What can geology tell us about the climate sensitivity? The PALEOSENS Project, comprising a large group of climatologists and palaeoclimatologists (PALEOSENS Project Members, 2012) calculated that over the past 65 million years the climate sensitivity for a doubling of atmospheric CO2 was between 2.2–4.8°C, which agrees reasonably well with calculations for the modern climate system by the Intergovernmental Panel on Climate Change (IPCC) (Solomon et al., 2007).
With rising emissions of CO2, geology tells us that that the planet will continue to warm, driving a long-term rise in sea level (Clark et al, 2016). The rise of sea level will take much longer to reach equilibrium than the rise in surface air temperature. For example, although warming due to orbital effects ceased 11,700 years ago, sea level continued to rise by 45m over the next 5,000 years (Clark et al., 2016). We would not expect such a large rise in the next few hundred years, because we no longer have large northern hemisphere ice sheets. But we have begun a process that will inevitably continue, melting parts of Greenland, Antarctica, and mountain glaciers, and affecting coastlines in the millennial time frame.
In summary, Earth’s history confirms that CO2 does change temperature, and that temperature does change CO2, and that we cannot blame the Sun for where we are now. All that we can blame are our emissions. Let’s get on with improving the long-term chances for survival of our species. Alarm is misplaced, but we should be seriously concerned about what global warming driven by our emissions will do to the climate that our children and their children will inherit.
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