GLOBAL ENVIRONMENTAL CHANGE - CAUSES AND EFFECTS
Health workers are not climate scientists. But understanding global environmental change and its impacts on physical and social systems is already important for clinical practice around the world, and this importance will only increase throughout the coming century. Clinical workers are used to communicating the effects of perturbations on complex systems in accessible ways; they make hard decisions in the face of uncertainty every day; and they render probabilistic reasoning tangible to the people whose lives it will affect. All this makes them ideally suited to communicating the evidence and causes of climate change, to peers and in public.
The Intergovernmental Panel on Climate Change (IPCC) works to assemble the state of the art in climate science to guide national and international adaptation and mitigation policy. Their 5th, most recent, Assessment Report (AR5) is the biggest systematic review of scientific work ever performed. It carefully analyses the best evidence in climate science, both the observational data that describes the history and present state of our environment, and modelling studies that allow us to predict how it will evolve. The majority of the content here is drawn from this report; for more details, the UK Royal Society and US National Academy of Sciences have produced an excellent summary on the evidence for anthropogenic climate change, and our friends at Medact provide this accessible précis of the climate science content of AR5 targeted at health workers.
The Intergovernmental Panel on Climate Change (IPCC) works to assemble the state of the art in climate science to guide national and international adaptation and mitigation policy. Their 5th, most recent, Assessment Report (AR5) is the biggest systematic review of scientific work ever performed. It carefully analyses the best evidence in climate science, both the observational data that describes the history and present state of our environment, and modelling studies that allow us to predict how it will evolve. The majority of the content here is drawn from this report; for more details, the UK Royal Society and US National Academy of Sciences have produced an excellent summary on the evidence for anthropogenic climate change, and our friends at Medact provide this accessible précis of the climate science content of AR5 targeted at health workers.
What's happening?
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The most publicised metric of our changing climate is 'global warming' - the planet is getting hotter. Surface temperatures are rising, with 14 of the 15 hottest years on record having occurred since the turn of the century. Average temperatures have increased an estimated 0.89C in the period from 1901-2012, with 0.72C of this rise occurring since 1951; the NASA time lapse video on the left shows the time course and geographical distribution of this temperature anomaly (or listen here if you prefer your temperature data in the medium of string quartet!). Rising temperatures are associated with disturbances in a range of other climatic parameters - sea ice is shrinking, snow cover falling, and sea levels rising.
The cause of this accelerated warming? According to the IPCC, it's virtually certain (>99% probability) that it's down to human influence. The primary mechanism is the release of greenhouse gases (GHGs) - gases which, when added to the atmosphere, insulate the planet against radiative heat loss. The most widely-known of these is carbon dioxide (CO2), but other important (though shorter-lived) contributors include methane (CH4) and nitrous oxide (N2O). |
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How do we know?
Palaeoclimate temperature data against atmospheric CO2 concentrations, showing strong historical correlation and our present unprecedented CO2 concentrations (click for source)
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As health workers are all too aware, causal attribution in complex systems can be a thorny issue, particularly when the political consequences of causal claims may be unpalatable to some. But climate scientists can be confident of their claims about the existence of warming, and its causes for the same reasons that medics can confidently assert that smoking causes lung cancer. To demonstrate this, consider climate change in relation to how epidemiologists justify that claim - the Bradford-Hill Criteria.
Basic physics provides a plausible mechanism for GHG emissions causing climate change, one that is supported by the strong association with appropriate cause-effect temporal ordering and an obvious dose-response effect demonstrated in the graph of palaeoclimate data against CO2 concentrations opposite. Perhaps most importantly, there is consistency between independent lines of evidence all demonstrating the same effect - observational data, 'fingerprinting' of different climate influences, and modelling studies with and without anthropogenic influence, all point to human activities shaping our climate. The last of these, climate models, also give us an impression of how the system will continue to develop. |
What does the future hold?
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If modelling the present climate is complicated, predicting its future development is an almost unimaginably messy business. Not only is the climate non-linearly dependent on a wide range of factors including, but not limited to, GHG emissions; those factors themselves are also subject to a huge array of social, political, economic and physical determinants. Population growth, development, energy transition, shocks to social systems like wars or economic crises - all of these will have a huge impact on our global emissions trajectory and resulting temperature changes. However, the climate models that allow us to quantify the influence of anthropogenic factors on the present climate also permit us to explore how different human-produced perturbations will influence it in the future. In the TED talk opposite, NASA scientist Gavin Schmidt explains how climate models work and what they allow us to predict; the bottom line is that, if we don't take prompt and radical action now, by the end of this century our environment will be unrecognisable from the one we currently inhabit.
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Global temperature changes according to different emissions scenarios (source: IPCC AR5 WGI)
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The IPCC describes its predictions in terms of a series of 'representative concentration pathways' - emissions trajectories that the world might follow on different assumptions regarding population and economic growth, mitigation policy, and other factors. On the highest, 'business as usual' pathway, surface temperatures will rise by an expected 2.6-4.8C (from 1986-2005 levels) by 2100; the lowest emissions pathway, however, will see only a further 0.3-1.7C warming.
To put ourselves on this lower trajectory would require carbon emissions to peak before 2020, followed by a rapid decline to net-zero emissions in the latter part of the century. If instead they continue to rise at current rates, we will remain firmly on course for 3C or more of warming. Achieving a zero-carbon society is not impossible, even for highly-industrialised economies like the UK; but it requires the political and economic will to take prompt and radical action. |
Does a few degrees of warming matter?
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Surface temperature increase alone, however, may not seem to be a great cause for concern. However, like human bodies, the climate has a narrow temperature window within which a 'safe operating space' for human society exists. Temperatures have risen 4-5C since the last ice age; we are on course for a similar magnitude of temperature change in the opposite direction, at a rate more than ten times as fast. We can expect even greater ecosystems upheaval as a consequence.
The IPCC summarises the effects of temperature change according to five key 'regions of concern', depicted in the ember diagram to the right. The unique and threatened systems are isolated physical, ecological or social systems (such as coral reefs, or Indigenous communities in the Arctic circle) that may be irreparably damaged without major global ramifications; as the graph demonstrates, many of these systems are already under threat. Extreme weather events refers to the range of climate-sensitive 'natural' disasters like floods, droughts, and hurricanes - like the 2013 Typhoon Haiyan/Yolanda that killed over 6300 people in the Philippines, provoking this moving speech at the UN climate talks in Warsaw from Yeb Sano, their representative. Global aggregate impacts concerns the aggregate harms felt worldwide from climate disruption; these are distributed unevenly, with the worst damages hitting those regions least able to cope and historically least responsible for CO2 emissions. Lastly, at temperatures beyond 3C we see a major risk of large-scale singular events - so-called 'tipping points' like ice sheet collapse - that cause major planetary disruption at a single stroke. Furthermore, many of these cause positive climate feedbacks - Siberian tundra melt, for instance, releases methane trapped in the permafrost that itself is a potent GHG - that could lock in catastrophic warming independently of our mitigation efforts. These are considered individually low-risk events, but are inherently unpredictable. |
Risk change with temperature increase for five 'regions for concern'. (Source: IPCC AR5 WG2)
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Beyond temperature - planetary vital signs in the Anthropocene
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When assessing an unwell patient, body temperature is important - but it doesn't tell the whole story. It is just one of many 'vital signs' that give an impression of their health. Similarly with the planet, temperature is only one indicator of the upheaval human societies are causing to the physical environment and ecosystems upon which our health and wellbeing depends. It is more accurate to think of climate change as symptom rather than disease - the latter is rather a pathological mode of living dependent upon unconstrained resource extraction and exploitation, to the detriment of people and planet alike. This condition has a range of other symptoms too - ocean acidification, biodiversity loss, air pollution to name a few - all driven by human activity.
As the picture to the left demonstrates, we are already exceeding several of these boundaries, while others remain under threat. The mechanisms driving these changes are not independent: the CO2 emissions causing climate change also lead to ocean acidification, while industrial agricultural systems dependent on energy-dense fossil fuels for their support also disrupt the nitrogen cycle with their insatiable demand for fertiliser and accelerate land use change, clearing forests for more pastoral land - thus removing our greatest carbon sinks and further accelerating climate change. |
The 'planetary boundaries' framework of the Stockholm Resilience Centre (source: see link)
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The environmental determinants of health - the physical and biological underpinnings of functioning human societies - serve to define a set of 'planetary boundaries' beyond which it will be impossible to maintain current ways of living. Human activity is the overwhelming factor threatening to move us beyond these boundaries, leading some to describe our current geological era as being defined by what we do to it - the 'Anthropocene'. This interactive map from the Smithsonian allows you to explore the diverse impacts humans are having on our environment, while Johann Rockstrom of the Stockholm Resilience Centre describes how the consumption-based economy driving these changes threatens our continued existence in the video to the left.
We are now living in the Anthropocene - and the way we are shaping our environment has profound impacts on human health. To learn more about these, read the next section. |
Further resources
Coming soon!
