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Climate change: the evidence is clear – now for the action

Volume 5 Number 1 - April 2008

Barrie Pittock

French title: Changement climatique Les preuves sont claires – Il est temps d’agir

Spanish title: El Cambio climático La evidencia esta clara – Ahora a la accion


The Intergovernmental Panel on Climate Change (IPCC) in 2007 finally acknowledged that humans are at the root cause of climate change. Evidence abounds that the world is heating up, but how quickly and what the impacts will be are open to speculation. The 10 areas of evidence presented here suggest that climate change is happening at a faster rate than anticipated. Considered collectively, the prospect is frightening and it is likely that we will face more serious outcomes than we currently understand. The time for action at all levels is now. Botanic gardens have an acute understanding of the impact of climate change on plant diversity. This needs to be articulated loudly and clearly to the millions of people who visit botanic gardens each year.


One of the greatest challenges we face this century is adapting to and mitigating climate change. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report was contributed to by over 2,000 scientists and is a manifestation of our global concern about this issue. It has taken a long time to emerge, but there is now consensus that ‘there is little doubt that climate change is happening and the warming of the world…is largely due to human emissions of greenhouse gasses’ (IPCC, 2007). The Fourth Assessment Report makes for worrying reading, yet many are concerned that it masks what a growing number of scientists believe are even more serious risks and implications about climate change.

Inevitably there are uncertainties in climate change science. They arise from questions of data quality, inadequate understanding of the climate system and its representation in climate models as well as uncertainties about future emissions of greenhouse gases resulting from socio-economic and technical developments. Policies therefore must be based on risk management. For example, we do not insure our house for the coming year because we are certain it will burn down, but because there is a small chance that it might and this would result in serious consequences for our finances. Better flood protection for New Orleans should have been built before 2005 (Fischetti, 2001), not because it was certain New Orleans would be flooded in 2005, but because it might have been.

Below, I highlight ten areas of concern based on observations and modelling studies. Looked at collectively, I argue that they suggest we are at greater risk of more serious outcomes than we currently understand – this makes it all the more critical for urgent action at local, regional, national and governmental level. This is something I will mention later in relation to botanic gardens.

Evidence suggesting more rapid climate change

1) The climate sensitivity may be larger than has been traditionally estimated

Recent estimates of climate sensitivity (the global warming after a doubling of the preindustrial carbon dioxide concentration) suggest a range of around 2ºC to 6°C (Hegerl et al., 2006) and a much higher probability of large global average surface warmings by 2100. Without a policy of drastically reducing greenhouse gas emissions, warming by 2100 would likely exceed 3.0ºC above pre-industrial , which is well above what many scientists consider would lead to ‘dangerous’ climate change impacts (Schellnhuber et al., 2006).

2) Global dimming is large but decreasing

Atmospheric particles (aerosols) reduce the amount of sunlight at the Earth’s surface. The resulting ‘global dimming’ has delayed warming of the oceans (Delworth et al., 2005), especially in the Northern Hemisphere. With stricter controls leading to reductions in emissions of particles and precursor compounds (Wild et al., 2005), there will be a decrease in the cooling influence these aerosols are having on the climate.

Given that the highest aerosol loading is in the Northern Hemisphere, reductions in global dimming are likely to have asymmetric effects, leading to greater warming in the Northern Hemisphere and to changes in cross-equatorial flows such as the Australian monsoon (Rotstayn et al., 2007) and the circulation in the Atlantic Ocean (Cai et al., 2006). By contrast, emissions of carbon dioxide (CO2) and other greenhouse gases exert a long-term warming influence because of their long lifetimes and the resulting cumulative effect on their concentrations. As a result, reductions in global dimming will lead to greater warming in the short term even if the emissions of greenhouse gases are cut back.

3) Permafrost melting and albedo changes

Observations show rapid melting of permafrost, or frozen ground (Arctic Climate Impact Assessment, 2004), which is expected to increase (Lawrence and Slater, 2005). Melting reduces the reflectivity, or albedo, of the surface (Chapin et al., 2005), and will likely lead to emissions of CO2 and methane previously stored in frozen soils. These are positive feedback (or amplifying) effects that may have been underestimated. Where permafrost is replaced by swampland, methane is likely to be emitted, but where it is replaced by dry soil, CO2 is more likely to be emitted.

4) Biomass feedbacks are kicking in

Saturation of terrestrial carbon sinks, and potential destabilization of large biospheric carbon pools are possible (Canadell et al., 2007). Observations of soil and vegetation acting as sources rather than sinks of greenhouse gases (Raupach et al., 2006) suggest an earlier-than-expected (Matthews et al., 2005) positive feedback in the terrestrial carbon cycle (Scheffer et al., 2006). Other factors that may lead to a more rapid global warming include reduced sequestration of root-derived soil carbon (Heath et al., 2005), overestimates of responses to ambient CO2 increases (Kilronomos et al., 2005), and forest and peat fires (Page et al., 2002) exacerbated by land clearing and draining of swamps.

Present indications are that emissions, sea level rise and global surface temperatures are all tracking along the highest of the range of estimates from the IPCC’s Third Assessment Report (Rahmstorf et al., 2007).

5) Arctic sea ice is retreating rapidly

Rapid recession of Arctic sea ice has been observed, leading to an acceleration of global warming as reduced reflection of sunlight increases surface heating (Wang et al., 2006). Some scenarios have the summertime Arctic Ocean becoming ice-free by the end of the century. There have also been longer seasonal melt periods, for the sea ice as well as the Greenland Ice Sheet and other land areas, especially since 2002. Serreze and Francis (2006) argue that the Arctic is presently in a state of ‘preconditioning’, setting the stage for larger changes in coming decades. They state that ‘extreme sea ice losses in recent years seem to be sending a message: the ice-albedo feedback is starting’. This has been borne out by record summer ice losses in 2007 (NSIDC, 2007), with some scientists now suggesting that the Arctic could be ice free in summer within a couple of decades.

6) Changes in air and sea circulation in middle and high latitudes

Different rates of warming at low and high latitudes in both hemispheres have led to increasing sea level pressure in the middle latitudes. This partly explains the observed and projected drying trends in winter rainfall regimes in Mediterranean-type climatic zones in both hemispheres (Pittock, 2003). This change has also strengthened the major surface ocean circulations, including the Antarctic Circumpolar Current (Fyfe and Saenko, 2006). These changes will significantly affect surface climate, including sea surface temperatures and storminess (Fyfe, 2003), and may already have accelerated melting in Antarctica (Marshall et al., 2006) and preconditioned the South Atlantic for the formation of tropical cyclones (Pezza and Simmonds, 2005).

7) Rapid changes in Antarctica

Rapid disintegration of ice shelves around the Antarctic Peninsula, and subsequent acceleration of outlet glaciers point to the role of surface meltwater in ice shelf disintegration (Dupont and Alley, 2006) and to the role of ice shelves in retarding glacier outflow. The Larsen B Ice Shelf collapsed spectacularly in 2002, following Larsen A and the Prince Gustav Channel Ice Shelf that both collapsed in 1995. Satellite observations clearly document that the sequence of the Larsen B Ice Shelf collapse involved the sudden disappearance of surface meltwater pools, followed immediately by the opening of crevasses and the break-up of the ice shelf over a period of a few weeks. Strengthening and warming of the Antarctic Circumpolar Current (Fyfe and Saenko, 2006) may accelerate Antarctic ice sheet disintegration by enhancing local warming, preventing sea ice formation, and undercutting ice shelves (Carril et al., 2005). Some indirect observations suggest that Antarctic sea ice extent is already in decline (Curran et al., 2004), although shorter direct observations are less clear.

8) Rapid melting and faster outlet glaciers in Greenland

The Greenland Ice Sheet is at a generally lower latitude than Antarctica and has widespread marginal surface melting in summer. The area of surface melting has rapidly increased in recent years, notably since 2002 (NASA, 2003, 2006). Penetration of this meltwater through moulins (crevasses and tunnels in the ice) to the lower boundary of the ice is thought to have lubricated the flow of ice over the bedrock and led to accelerated glacier flow rates (Thomas et al., 2006). Melting of tidewater glaciers from the bottom, pushing back the grounding line, may also be contributing to acceleration of flow (Bindschadler, 2006).

These observational results indicate mass losses considerably faster than were modeled by glaciologists using models which did not take account of the recently identified mechanisms of meltwater lubrication and tidewater glacier undercutting (Ridley et al., 2005). Simulations and paleo-climatic data indicate that Greenland and Antarctica together contributed several meters to sea level rise at 130,000 to 127,000 years ago, a time when global temperatures were about the same as presently projected for 2100 (Overpeck et al., 2006).

9) Tropical cyclones may already be more intense

Some observational analyses point to a rapid intensification of tropical cyclones over recent decades (Hoyos et al., 2006). However, modeling of tropical cyclone behavior under enhanced global warming conditions (Walsh et al., 2004) suggests only a slow increase in intensity that would not yet be detectable given natural variability. However, according to Pezza and Simmonds (2005), the first recorded South Atlantic hurricane may be linked to global warming.

10) Variations in the North Atlantic Ocean circulation and salinity

The North Atlantic has a complex current system, with the largely wind-driven Gulf Stream splitting into the North Atlantic Current that heads north-east into the Norwegian Sea, and a subtropical recirculating arm, known as the Azores and Canary Currents, which turns south. Relatively warm, but highly saline, surface water in the northern arm tends to sink to a depth of several kilometers in three regions – the Labrador Sea, south of Iceland and between Greenland and Norway. The north-flowing arm transports heat from low latitudes to high latitudes, tending to warm northwestern Europe. Bryden et al. (2005) reported a significant slowing of this regional sinking, or ‘meridional overturning’ circulation, but it may be natural variability (Bindschadler, 2006). Cai et al. (2006) and Delworth and Dixon (2006) suggest that without the ‘protective’ effect of aerosols the slow-down would be 10% greater, indicating further slow-down as aerosols decrease. Any slowdown now or in the future is likely to be related to freshening of surface waters in the Arctic Ocean due to increased precipitation and river inflow and the recent increase in ice-melt from Greenland and other glaciers (Swingedouw, 2006; Pittock, 2008).

We have the evidence, so what’s next?

The above lines of evidence (supported by well over 100 recent scientific papers – see Pittock, 2008), while not definitive and in some cases controversial, suggest that the balance of evidence may be swinging toward more extreme global warming and sea-level rise outcomes. While some of the observations may be due merely to natural fluctuations, their conjunction and in some cases positive feedbacks (from permafrost melting, biomass changes, Arctic sea ice retreat, and melting of Greenland) are causes for concern. They highlight the need for us to consider the whole system, not just individual parts in isolation.

The observations and linkages suggest that critical levels of global warming may occur at even lower greenhouse gas concentrations and/or anthropogenic emissions than was considered justified in the IPCC (2007) report. The observed changes in Greenland and Antarctica suggest that a more rapid rise in sea level may be imminent, as has been observed in recent years. Indeed, Rahmstorf et al. (2007) find that emissions, global surface temperature and sea level rise are all increasing at rates at the very highest end of the IPCC range. These recent developments increase the urgency of further improving climate models, and of taking action to reduce emissions in order to avoid the risk of unacceptable levels of climate change.

What does this mean for botanic gardens?

As guardians of plant diversity, botanic gardens have an acute understanding of the impact of climate change on plant diversity. According to BGCI’s website (BGCI, 2008):

  • Plants will be unable to change their distribution fast enough and, those with long life cycles and/or slow dispersal mechanisms, will be particularly vulnerable.
  • Isolated plant species such as alpine species and island endemics will also be vulnerable as they will have nowhere to move to.
  • Coastal species will be ‘squeezed’ between human settlements and rising sea levels
  • Plant genetic composition may change in response to the selection pressure of climate change
  • Some plant communities or species associations may be lost as species move and adapt at different rates
  • Increased invasions by alien species may occur, as conditions become more suitable for exotic species while native species become less well suited to their environment.
  • Many plant communities act as ‘sinks’ which helps to offset carbon emissions. However over the next 70 years, the effects of climate change on plants mean many terrestrial sinks may become sources.

With knowledge comes responsibility. Botanic gardens have a responsibility to provide evidence and to explain to the public about the possible dangerous or unacceptable outcomes of climate change in relation to plant diversity, no matter how alarming or extreme these outcomes may be, and even if the probability of any of them occurring is small. Botanic gardens exist in most countries of the world and through their education and public awareness programmes collectively reach millions of people. They clearly have a significant contribution to make in combating and mitigating climate change.


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Le Groupe d’experts intergouvernemental sur l’évolution du climat (GIEC) a finalement reconnu en 2007 que les humains sont la cause du problème du changement climatique. Les preuves que le monde se réchauffe sont très nombreuses, mais à quelle vitesse et quels en seront les impacts sont des questions qui demeurent ouvertes à la spéculation. Les 10 éléments de preuves présentés dans cet article supposent que le changement climatique survient plus rapidement que prévu. D’un point de vue global, la perspective est effrayante et il est probable que nous soyons confrontés à de graves conséquences dépassant toute compréhension actuelle. Le moment d’agir à tous les niveaux est aujourd’hui et maintenant. Les jardins botaniques ont une profonde compréhension de l’impact du changement climatique sur la diversité végétale. Ceci doit être clairement formulé haut et fort aux millions de personnes qui visitent chaque année les jardins botaniques.


El Grupo Intergubernamental de Expertos sobre el Cambio á (IPCC) finalmente reconoció en el 2007 que el ser humano es la causa del cambio climático. Abunda la evidencia que la temperatura del mundo aumenta, pero aun se especula sobre a que ritmo y con que posible impacto. Las 10 áreas de evidencia que se presentan aquí indican que el cambio climático está ocurriendo a una velocidad mas elevada de la anticipada. Si se consideran colectivamente, el prospecto es alarmante y es probable que habrán resultados mas serios de los que actualmente podemos comprender. Esta es la hora de tomar acción a todos los niveles. Los jardines botánicos tienen un profundo conocimiento del impacto del cambio climático sobre la diversidad vegetal. Esto se tiene que proclamar con fuerza y claridad a los millones que visitan los jardines botánicos cada año.

Barrie Pittock
33 Bourneville Avenue, Brighton East Vic 3187, Australia

Dr A. Barrie Pittock is one of the world’s leading scientists in atmospheric research and the author of over 200 scientific papers. He was a senior scientist with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) for over 30 years where he led the Climate Impact Group in the 1990s and is currently an Honorary Fellow at CSIRO. In 1999 he was awarded an Australian Public Service Medal for his leadership and visionary approach to identifying, researching and communicating a range of global climate science issues.

This paper is based on a book chapter by Pittock (2008, chapter 1 pp. 11-27) and has been edited by Julia Willison, BGCI, who contributed comments relating to botanic gardens.

MacCracken, M. C., Moore, F. and Topping, J. C. Jr. (editors), 2008: Sudden and Disruptive Climate Change: Exploring the Real Risks and How We Can Avoid Them, Earthscan Publishers, London, UK, 320 pp.