The concept of an environmental tipping point was first broached by James Hansen in reference to climate change. Hansen is an Adjunct Professor of Earth and Environmental Sciences at Columbia University’s Earth Institute and recently retired climate scientist for NASA’s Goddard Institute for Space Studies. In 2005 at a presentation to the American Geophysical Union (AGU), Hansen stated, “we are on the precipice of climate system tipping points beyond which there is no redemption.”1 His use of the term “tipping point” to warn against the dangers of irreversible climate change started a trend in climate change communication that was picked up by the media and which they used extensively to report on the growing body of evidence that man-made activities were causing a significant increase in greenhouse gas levels in the atmosphere.
But Hansen was not the first to discuss climate change in terms of tipping points. In 2003, Alley et al. defined a “tipping point” as an event that “occur[s] when the climate system is forced to cross some threshold, triggering a translation to a new state at a rate determined by the climate system itself and faster than the cause.”2 The climate system being referred to encompasses the atmosphere, oceans, land, the cryosphere, and the biosphere.
The term eventually came to be used widely in other areas of science by biologists, marine scientists, engineers, and even policy makers. In 2008, a group of scientists lead by Johan Rockström proposed the idea of planetary boundaries that “define the safe operating space for humanity with respect to the Earth system and are associated with the planet’s biophysical subsystems or processes.” They identified nine such Earth-system processes that, if crossed, “could generate unacceptable environmental change.”3
- Climate change;
- Rate of biodiversity loss (terrestrial and marine);
- Interference with the nitrogen and phosphorus cycles;
- Stratospheric ozone depletion;
- Ocean acidification;
- Global freshwater use;
- Change in land use;
- Chemical pollution;
- Atmospheric aerosol loading
But rather than call them tipping points, Rockström and colleagues referred to the planetary boundaries of these processes as “critical thresholds.” Furthermore, Rockström et al. believed that three of these processes have already crossed the critical threshold or tipping point: climate change, rate of biodiversity loss, and interference with nitrogen and phosphorous cycles.
As a contributor to climate change, most scientists agree that the highest carbon dioxide concentration that can be considered safe is 350 ppm.3,4 This threshold is based on paleoclimate data from the last 100 million years, which showed that not only was carbon dioxide a major contributor to global cooling, but that ice sheets did not form on Earth when CO2 concentrations were at about 450 ppm (±100 ppmv), suggesting the presence of a critical threshold. Setting the lowest possible threshold as the safe level would ensure that the large polar ice sheets will not disappear.3 The concentration of carbon dioxide stood at 395 ppm globally in 2012, with the Arctic recording the highest ever at more than 400 ppm and Alaska, Greenland, Iceland, Norway, and Mongolia also recording 400 ppm or higher. The last time CO2 concentrations were this high was 800,000 years ago.4
In terms of biodiversity loss, Rockström et al. said that the rate of species extinction today is 100 to 1000 times more than the natural rate (0.1 to 1 million extinctions per million species per year for marine life and 0.2 to 0.5 extinctions per million species per year for mammals) based on fossil records. The rapid extinction has been due largely to land use changes such as the conversion of wildlife habitats for agriculture and urban uses.
Natural nitrogen and phosphorus cycles have been disrupted by man-made additions of nitrogen and phosphorus through agriculture. Nitrous oxide, a by-product of this man-made process is one of the greenhouse gases causing climate change and phosphorus inflows into oceans 20% greater than natural levels have caused past ocean anoxic events.3
The interrelatedness of the various climate systems and recent scientific evidence have led a group of scientists led by Anthony Barnosky to make similar conclusions that the Earth is on the brink of a massive global-scale tipping point due largely to human-induced climate change, which they detailed in an article published in Nature on June 2012.5 Whereas most scientists have documented localized or ecosystem-specific tipping points, Barnosky et al. warn that a planetary-scale critical threshold could soon be breached as more localized tipping points fall due to human activities. They propose the need to create improved biological forecasts that can detect localized and global-scale “critical transitions.”
In its 2007 Fourth Assessment Report, the International Panel on Climate Change (IPCC) discussed the issue of tipping points, referring to them as “major or abrupt climate changes.” They identified six climate and climate-affected systems that have the possibility of undergoing such abrupt changes or experience irreversible consequences:6
Atlantic Meridional Overturning Circulation (AMOC) and other ocean circulation changes;
- Arctic sea ice;
- Glaciers and ice caps;
- Greenland and West Antarctic Ice Sheets;
- Vegetation cover; and
- Atmospheric and ocean-atmosphere regimes.
In 2008, 52 of the world’s leading experts on the climate attended a workshop called “Tipping Points in the Earth System.” In a report published by Lenton et al in 2008 in Nature,7,8 the participants identified nine climate systems that have separate tipping points in danger of crossing the threshold due to the increased levels of carbon dioxide and global temperature:
- Arctic Sea-Ice;
- Greenland Ice Sheet;
- West Antarctic Ice Sheet;
- Atlantic Thermohaline Circulation (a component of the Atlantic MOC);
- El-Niño-Southern Oscillation;
- Indian Summer Monsoon;
- Sahara/Sahel and West African Monsoon;
- Amazon Rainforest; and
- Boreal Forest
Scientists have identified several paleoclimatic events as evidence that climatic tipping points have been breached in the past. The most pertinent of these events is the rise in sea levels as ice sheets melted and the corresponding temperature for the geologic time periods in which they occurred.9 For example, scientists believe that the global sea level was 4 to 6 meters higher during the last interglacial period compared to today based on the presence of elevated beaches. During that period, about 125,000 years ago, global average temperature was about 1°C warmer than the current interglacial period. Data from ice core sheets also suggest that the retreat of the Greenland Ice Sheet and other ice fields in the Arctic may have contributed to a rise in sea levels of about 2 to 4 meters during the last interglacial period. Scientists warn that the IPCC’s estimate of a 2 to 4°C increase in global temperatures in the next 100 years could lead to a rise in sea levels of 10 to 20 meters or more mainly from the ice sheets in Greenland and West Antarctic. It is, however, uncertain how long it will take for the ice sheets to melt: it could take decades or centuries or longer.
There are still disagreements regarding the concept of environmental tipping points, mainly due to lack of quantitative measures to determine when exactly a critical threshold is breached for each climate system. Many of the assumptions currently being used to estimate thresholds are based on the human (in)capacity to respond to such dangers. There are those who believe that tipping points are simply expressions of anxiety that humans may not be able to solve climate change problems once the thresholds have been breached. Still others believe that there is enough scientific evidence for tipping points that cannot be ignored and thus require immediate action.
- Hansen, J.E. (2005). Is there still time to avoid ‘dangerous anthropogenic interference’ with global climate? Retrieved from: http://www.columbia.edu/~jeh1/2005/Keeling_20051206.pdf
- Alley R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke, R.A,… Wallace, J.M. (2003). Abrupt climate change. Science, 299(5615), 2005–2010.
- Rockström, J., Steffen, W., Noone, K., Persson, Å., Chapin, F. S. III, Lambin, E. F.,…Foley, J.A. (2009). A safe operating space for humanity. Nature, 461, 472–475. doi:10.1038/461472a
- Borenstein, S. (2012, May 31). Scientists: Carbon dioxide at highest level in 800,000 years. USA Today. Retrieved from: http://usatoday30.usatoday.com/weather/climate/globalwarming/story/2012-05-31/carbon-dioxide-greehouse-gas-level-climate-change-global-warming/55312242/1
- Barnosky, A.D., Hadly, E.A., Bascompte, J., Berlow, E.L., Brown, J.H., Fortelius, M.,…
- Smith, A.B. (2012). Approaching a state shift in Earth’s biosphere. Nature, 486, 52–58. doi:10.1038/nature11018.
- IPCC (Intergovernmental Panel on Climate Change). 2007a. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S. D.Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp. Retrieved from: http://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htm
- Lenton, T.M., Held, H., Kriegler, E., Hall, J.W., Lucht, W., Rahmstorf, S., Shellnhuber, H.J. (2008). Tipping elements in the Earth’s climate system. Proc. Natl. Acad. Sci., 105(6), 1786–1793. Retrieved from: http://www.pnas.org/content/105/6/1786.full.pdf
- Lenton, T.M., Livina, V.N., Dakos, V., Van Ness, E.H., Scheffer, M. (2012). Early warning of climate tipping points from critical slowing down: comparing methods to improve robustness. Phil. Trans. R. Soc. A. 370, 1185–1204. doi:10.1098/rsta.2011.0304
- Climate Tipping Points: Current Perspectives and State of Knowledge. (2009, July). U.S. Department of Transportation. Retrieved from http://climate.dot.gov/about/overview/pdf/climate_tipping_points.pdf