The world has relied far too long on global temperature as the single indicator of climate change. With the increasing capacity of satellites to provide comprehensive data on the Earth system, it is time for a new approach – a basket of indicators that gives a more complete picture of climate change and its impacts.
Elsewhere in this Handbook you will see described the Essential Climate Variables (ECVs), a complex set of physical quantities that drive the processes that govern climate change on Earth. They were developed by GCOS for the purposes of scientific understanding, modelling and prediction of the world’s climate. Some, such as albedo, need a complex discussion of the Earth’s radiation balance before their meaning can be conveyed to non-specialists and hence are not always suitable indicators of climate change in a wider context.
Are there a few direct observations that we can make which would give a better picture of change in the Earth system, without necessarily requiring a deep understanding of the physical processes involved? And which do not encourage simplistic interpretations? Sea level rise is already a good indicator. Scientists agree on its meaning, and policymakers and the public can visualize its impacts. Global surface temperature is at first sight a straightforward indicator of climate change and policymakers make use of the concept in negotiations, but science tells us it is an ambiguous indicator. It says something about the weather, since the production of water vapour is a sensitive function of sea surface temperature, but it does not tell you how fast climate is changing despite its frequent use as a reference.
The public controversy on the so-called “hiatus” is a case in point. Had the public been made aware instead of ocean heat content data, it would have seen there was no hiatus in the rate that the energy in the climate system was increasing. Even as policymakers have set temperature as the single most prominent metric for policy success, it is one of the worst real indicators of actual stress.
It is often said that the public will not pay attention to more than one indicator. We think this underestimates the public – it also underestimates policymakers who regularly deal with complex policy challenges, such as employment and economic growth, for which they must rely on many indicators. We would not expect to have a thermometer thrust into our mouths in hospital and have no other investigations of our body to guide a complex differential diagnosis of many possible maladies. We know we can have cancer without running a temperature. We are used to doctors measuring more than one vital sign. We expect to have blood assays, ECGs, tests for liver function, kidney function or others, depending on our symptoms. When we are faced with a serious medical problem, we want them all. Why not treat the planet’s health the same way? Why not vital signs for the Earth?
Planetary vital signs should be reliable, simple, have a long term history, reflect a range of possible symptoms of climate change, integrate as far as possible many wider effects of change, be easily interpreted and conversely difficult to misinterpret, either by accident or intent. They need not be the same for policymakers and the public, but they must be accessible to both.
Figure 1: Total solar irradiance – Annual average composites of measured total solar irradiance: The Active Cavity Radiometer Irradiance Monitor (ACRIM) (Willson and Mordvinov, 2003), the Physikalisch-Meteorologisches Observatorium Davos (PMOD) (Frohlich, 2006) and the Royal Meteorological Institute of Belgium (RMIB) (Dewitte et al., 2004). These composites are standardized to the annual average (2003–2012) Total Irradiance Monitor (TIM) (Kopp and Lean, 2011) measurements that are also shown.
Credit: IPCC AR5
Figure 2: Atmospheric CO2
Credit: Scripps, NOAA
Figure 3: Sea level rise – Multi-mission global sea level trend from altimetry (1993-2010)
What might be candidates? There are several possible lists already in existence – this idea is not new – and the GCOS ECVs themselves constitute some excellent candidates. However, we should try to define a small set of maybe 6–8 series of observations that reflect in a straightforward manner the state of the Earth’s climate. That said, the subject is not simple – hence the need for the fifty ECVs – but we are not here trying to explain the processes involved, but to look for the consequences.
Figure 4: Energy accumulation within the Earth’s climate system
Credit: IPCC AR5
Figure 5: Global average surface temperature anomaly
Credit: UK Met Office Hadley Centre
Candidates for such indicators might be:
− Surface temperature – for all its faults, this is one sign with which many are already familiar and we should retain it, but not alone.
− Sea level is an excellent integrator of many aspects of change – ocean warming, mass loss from the ice sheets, glacier melting and groundwater changes. Half the world’s population lives within 100km of a coast, and will be affected by sea level rise. Some small island states and coastal regions are at particular risk.
− Atmospheric carbon dioxide – the fundamental driver of anthropogenic climate change. We now monitor from space the spatial distribution of atmospheric carbon dioxide worldwide, but we have over 50 years of observations of global CO2 content from Mauna Loa. Both the CO2 emission rate and the cumulative emissions are important to policy.
− The energy content of the climate system – largely in the form of Ocean Heat Content (OHC). IPCC AR5 tells us that 93% all the energy increase in the climate system is going into the oceans. This is of greatest importance as OHC is the fundamental driver of the climate system (not a consequence, like surface temperature) and the reservoir of energy that will drive future weather systems.
− Total solar irradiance – we need to see whether changes in the Earth system are driven by changes in the solar output, independently of anything humanity is doing to the planet.
− Measures of extreme climate event occurrence and intensity – one place to start is an index that measures the total global area during the year in which extreme conditions that depart by three standard deviations from the local and seasonal mean occur.
− Arctic sea ice extent and volume – we now have a very reliable means to measure from space the extent (area) of winter and summer Arctic ice, and more recently also to measure its thickness, and hence to calculate the total ice volume at any given time. This record extends in part back to 1979.
Satellites are critical in providing the observations of many of the above. For example, arctic sea ice extent has been measured since the 1970s by passive microwave imaging systems and for the last 20 years or so by active radar imaging sensors also. In addition, dedicated satellites (Icesat, Cryosat) with specific relevant technologies have allowed measurements not only of the extent, but also the thickness and hence total volume of sea ice to be made.
Sea level is now measured most accurately, and globally, by a series of precise altimeters. These measurements go back over 20 years and will continue into the future due to the Sentinel 6 series of missions. Total solar irradiance is best measured from above the Earth’s atmosphere by missions such as the Solar and Radiation Climate Experiment (SORCE). Satellite observations contribute significantly to all the other parameters also.
We could think of more candidate indicators, but we should restrict ourselves, for the sake of simplicity and better communication, to a rather small number such as the above. The climate science community has been generating and using indicators for its own purposes for several decades. It is now time to put this experience to broader use. An indicator is simply a number unless people make use of it. The science community cannot do it on its own. It is essential that policymakers and public communicators participate in the choice, format and dissemination strategy for the world’s first set of planetary vital signs.
Let the dialogue begin. We hope that decision-makers call for it and we know that the scientific community can contribute a good panel of indicators if asked. A good first step might be a conference or series of workshops where natural and social scientists, policy specialists and stakeholders frame a proposal on how to start.
Figure 6: Satellite data supports the study of the long-term variability of polar sea ice cover and its relationship to climate change
Credit: NASA GSFC
Stephen Briggs (European Space Agency), Charles Kennel (Scripps Institution of Oceanography), David Victor (University of California at San Diego)