Tag: paleoclimate

Fall in San Francisco: Jim Renwick’s AGU report

Jim RenwickI took part in the 2015 AGU Fall Meeting, held at the vast Moscone Center in San Francisco, 14-18 December. As always, it was an absolute cornucopia of everything to do with the Geo/Earth Sciences, from exoplanets to the earth’s core to climate change and science policy, delivered by over 20,000 geoscientists. The Fall Meeting is always a blast, a real mind-expander.

This year, I was committed to chairing sessions first thing on Monday morning and then again on Friday. Monday’s session was “Evaluating Reanalysis: What Can We Learn about Past Weather and Climate?” with my sub-session having a focus on polar regions. The Thu/Fri session was “Precipitation over Mountainous Terrain: Observations, Understanding, Modeling, and Future Prospects”. In between, I soaked up as much as I could, wandering the halls to hear and see fascinating presentations on climate history, science communication, sea ice, and designing climate change musicals for primary school children. Here’s a few highlights, my personal “tip of the iceberg” from this year’s meeting.
Continue reading “Fall in San Francisco: Jim Renwick’s AGU report”

Getting it Wright on sea level rise

Sea level rise of up to 40cm around New Zealand by the middle of this century is already locked in and will cause significant problems for coastal communities and infrastructure, according to a new report just released by Dr Jan Wright, the Parliamentary Commissioner for the Environment. The report — Changing Climate and Rising Seas: Understanding the Science [pdf] — provides an overview of why sea levels are currently rising and why they are expected to continue rising over the rest of this century and beyond. A follow-up report due next year will “show in some detail which areas of the coastline around the country are most vulnerable to sea level rise and assess the risk to infrastructure in those areas”.

Introducing the report, Dr Wright said that the scientific evidence is now irrefutable. “The climate is changing and causing the sea to rise”.

“A rise of 30 cm may not sound much, but its impact will be very costly for many landowners. Damaging coastal floods will become increasingly frequent. The insurance industry is becoming aware of, and responding to, the increased flooding risk. Some councils and communities have already started to face hard questions.”

Commenting on the report for the Science Media Centre, Associate Professor Nancy Bertler of the Joint Antarctic Research Institute, Victoria University of Wellington/GNS Science, said:

The report provides an excellent summary on the current knowledge of past and future sea level rise including the main drivers and the regional patterns. Dr. Wright highlights the concern of the scientific community on the possibility of substantial and abrupt future contributions from the West Antarctic ice sheet.

Additional important considerations are that: worldwide over 200 million people live within one metre of sea level. The last time atmospheric carbon dioxide concentration was at 400 ppm (3-5 million years ago) the associated global temperatures caused the Greenland and West Antarctic ice sheets to catastrophically collapse – raising global sea level by around ten to twenty metres.

The rate at which sea level will rise has important implications on our ability to adapt. New research suggests that sea level could rise as quickly as 4 metres per 100 years (or 1 metre per 25 years). Assuming even a modest global sea level increase of 50 cm by 2100 (IPCC scenario RCP 4.5), the frequency of coastal inundation in New Zealand is predicted to increase by a multiplier of 1000 times.

Under such a scenario, an annual event becomes a daily event, a ‘100 year’ event occurs several times per year. As an approximation: every 0.1m rise triples the frequency of inundation events.

Dr Wright focusses on the near term implications for New Zealand, a sensible choice given the tendency to dismiss sea level rise as a problem for the distant future, but in my view she misses an opportunity to spell out the strong relationship between atmospheric CO2 levels and equilibrium sea level. The last time CO2 stood at 400 ppm, global sea level was about 20m higher than today. That’s where we’re heading, unless we can get greenhouse gas levels down, and it has very important implications for emissions policy. But I’m nit-picking…

Changing Climate and Rising Seas is a very readable introduction to the science of sea level rise, and gives a very clear picture of the state of current knowledge. It’s a welcome addition to what passes for national discourse on the inevitability of climate change and the necessity of adapting to what it brings. Next year’s report on regional impacts will be even more important.

Climate Change and the Course of Global History

The title in the Kindle Store was irresistible: Climate Change and the Course of Global History: A Rough Journey. American historian John L Brooke is the author, and the book is notable for its attempt to integrate climate science with the study of human history. In his acknowledgements the author brackets climate scientists with historians and archaeologists in the long list of people with whom he has corresponded and from whom he has received data and an understanding of scientific culture.

The scope of the book is as wide as human emergence in the evolutionary process, and before. It’s not my purpose to track through the long story the author has to tell or to follow the intricacies of the climate shifts he refers to. The book demands and rewards patient reading in these respects. But I offer a few comments arising from my reading of the book.

An historian making the effort to understand climate science as thoroughly as Brooke does seems to me in itself worth remarking. Often I could have been reading one of the many books by scientists or science writers that I’ve reviewed over past years on Hot Topic. The culture gap between the sciences and the humanities that C P Snow’s famous 1959 lecture lamented was certainly not evident in Brooke’s history. Five years ago I reported the plea of biographer Richard Holmes at the Hay Festival that we reject the notion of two cultures and accept the duty to understand the scientific discoveries of the modern age, a duty of crucial importance in the face of global warming. Brooke has clearly accepted that duty, though I fear many educated in the humanities continue to excuse themselves from it.

Continue reading “Climate Change and the Course of Global History”

A lecture not to miss

Tim Naish’s lecture, of which we gave notice recently, is now recorded on the Climate Change Research Institute’s website. I warmly recommend it for viewing. Naish is one of the lead authors for the paleoclimate chapter for working group 1 of the IPCC Fifth Assessment Report due in 2013. In this lecture he uses paleoclimate material to provide perspective for the projections of rising global temperature and climate change. We are headed for climates and temperatures that haven’t been seen on the planet for more than a million years and the paleoclimate record helps us to understand what we might expect in terms of polar ice behaviour and sea level rise.

In fact we have to go back 3 million years – to the mid-Pliocene –  before we see temperatures like those the models are projecting, 2 to 3 degrees warmer by 2100. The atmospheric CO2 level then was about 400 parts per million. This Pliocene warm period is becoming an important window into what we might expect incoming decades. Continue reading “A lecture not to miss”

The Long Thaw

The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate

The legacy of our release of fossil fuel CO2 to the atmosphere will be long-lasting. It will affect the Earth’s climate for millenia. We are becoming players in geologic time. That is the conclusion that climatologist David Archer shares with a general audience in his newly published book The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth’s Climate.

The author is a professor in the Department of The Geophysical Sciences at the University of Chicago and a contributing editor at Real Climate. His book is relaxed in style, almost conversational sometimes, repetitive on occasion, but nevertheless closely focused and packed with instructive detail. It was a pleasure for a non-scientist like me to read. He seems to understand how to illuminate processes for the general reader. For example, his chapter on the distribution of carbon in the atmosphere, the land and the ocean, and his explanation of the interactions between them in the carbon cycle, provided angles and information that pulled together satisfyingly the bits and pieces of my hesitant understanding.   Similarly what he writes about the acidifying of the ocean by CO2 and the part calcium carbonate plays in slowly neutralising its effect is a model of lucidity.

The book’s structure is simple.  There are three sections.  The first describes the situation we are in right now – meaning the 20th and 21st centuries.  The second section is about the past, investigated as a forecast for the future.  The final section looks into the deep future.

Archer produces no surprises about our current situation.  The basic physics of the greenhouse effect – that gases in the atmosphere that absorb infrared radiation could eventually warm up the surface of the earth – was described in 1827 by the French mathematician Fourier. Then in 1896 Swedish chemist Arrhenius estimated the amount of warming that the Earth would undergo on average from a doubling of the atmospheric CO2 concentration – what we now call the climate sensitivity. Such work sets the scene for the climate science which has exploded in the past few decades as global warming grew from a prediction into an observation.    He describes many aspects of our current understanding of global warming, with several particularly helpful sequences, such as that on the relative strengths of four external agents of climate change called climate forcings – greenhouse gases, sulfur from burning coal, volcanic eruptions, changes in intensity of the sun. The warming that is occurring cannot be explained by natural forcings.  Looking ahead in the present century he is very aware that sea level rise by 2100 may well be higher than predicted by the IPCC, as it begins to appear that the ice models used to forecast may be too sluggish to predict the behaviour of real ice.

In the second section he moves steadily back in time, starting with the last 100,000 years where the abruptness of some of the changes detected leads him to reflect that the IPCC forecast of a smooth rise in temperature from 0.5 degrees excess warmth today  to about 3.0 degrees excess warmth in 2100 represents a best-case scenario in that it contains no unfortunate surprises. He then treats the longer-term glacial climate cycles through the last 650,000 years, paying attention to orbital forcing and to the ups and downs of atmospheric CO2 through the cycles.  He envisages the ice sheets and CO2“entwined in a feedback loop of cause and effect, like two figure skaters twirling and throwing each other around on the rink.” His final step back is to the hothouse world of 50 million years ago and beyond that to transitions between hothouse and ice age climates over 500 million years. He selects the Paleocene Eocene Thermal Maximum event (recently discussed on Hot Topic) as an analogue for the global warming future.

The third section looks at that future.  In discussing the land’s and ocean’s ability to take up carbon being released from fossil fuels he considers it likely that there are limits to that process which will mean that a significant fraction of fossil fuel CO2 will remain in the atmosphere for millenia into the future.  There are calming effects from the carbon cycle, but there can also be opposite effects as seems likely to have been the case at times in the past.  Hopefully large scale methane hydrate release won’t be a large part of such feedbacks, but if the ocean gets warm enough it is possible and could double the long-term climate impact of global warming.

For now the carbon cycle is responding to the CO2 increase by inhaling the gas into the ocean and high-latitude land surface, damping down the warming effect. But on the timescale of centuries and longer the lesson from the past is that this situation could reverse itself, and the warming planet could cause the natural carbon cycle to exhale CO2, amplifying the human-induced climate changes.

The clearest long-term impact of fossil-fuel CO2 release is on sea level rise.  The book has a restrained chapter on this, but there is no escaping what will happen if the ice sheets melt. “We have the capacity to ultimately sacrifice the land under our feet.“

Have we averted an ice age?  Archer discusses this possibility, but finds the evidence uncertain.  He would in any case not put such a possibility forward as an argument in favour of CO2 emissions. All it means is that natural cooling driven by orbital variation is unlikely to save us from global warming – at this stage the much greater danger. Incidentally he mentions Ruddiman’s book Plows, Plagues, and Petroleum briefly and appreciatively in this section, but gives reasons for doubting its conclusions. (The book was reviewed on Hot Topic recently.)

In his epilogue on economics and ethics, where he ponders whether we are likely to turn away from the path we are currently on, he offers a comparison with slavery, another ethical issue: “Ultimately it didn’t matter whether it was economically beneficial or costly to give up. It was simply wrong.”

James Hansen describes the book as the best about carbon dioxide and climate change that he has read.  “David Archer knows what he is talking about.” To which I would add that he also knows how to explain it clearly to anyone prepared to give him reasonable attention.