Despite the great complexity of climate change, scientists must accept the challenge of communicating their findings to policymakers and the general public. They must do this in a way that is simple enough for non-specialists to understand, but not so simple that it distorts the science. Climate science is often communicated through well-written technical reports, but it can also be effectively explained through graphics, animations, oral presentations, compelling narratives, trusted messengers, and popular books and films.
A promising approach to conveying the reality of climate change is to develop indicators – numbers and scales that track the state or level of some aspect of the climate. One widely used indicator in climate science is the change in the global average temperature of the lower atmosphere. This indicator is also one of the targets set out by the 2015 Paris Agreement on climate change, which calls for keeping a global temperature rise this century to well below 2 °C above pre-industrial levels while pursuing efforts to limit the temperature increase even further, to 1.5 °C.
Indicators have a number of advantages. They are quantified, objective, based on data provided by virtually all countries, and they demonstrate change over time. This is why Agenda 2030, adopted by the United Nations in 2015, seeks to use indicators to track progress on the various Sustainable Development Goals, including SDG 13 on combatting climate change and its impacts.
The Parties to the United Nations Framework Convention on Climate Change (UNFCCC) are also likely to include indicators in the five-yearly “global stocktake” for measuring progress under the Paris Agreement. In addition to indicators that capture progress on mitigation, indicators can measure changes in the climate change impacts that should be targeted by adaptation efforts. The ongoing negotiations on how to structure this process and on what information to include in the stocktake are to be finalized before the first stocktaking exercise takes place in 2023.
Over the past year, WMO, together with partner organizations, including those in the Global Climate Observing System (GCOS) and the World Climate Research Programme (WCRP), has started developing a short list of “headline” indicators tracking changes in the physical climate system. The indicators discussed in this article focus on monitoring such changes. Additional indicators will also be needed to track climate risks and impacts in order to guide climate adaptation; this should be the focus of future work. WMO will remain fully committed to this effort.
An ideal shortlist
The large number of existing indicators produced by climate scientists – many of which draw on the 55 GCOS Essential Climate Variables – are useful for many specific technical and scientific purposes and audiences. They often use different sources and baselines, however, which are not always comparable. Many are also rather complex. They are thus not all equally suitable for helping non-specialists to understand how the climate is changing. The challenge is to identify a subset of key indicators that capture the essential features of climate change in a comprehensive way and that can be well understood by non-scientific audiences.
To ensure the clear and concise communication of global climate change, the number of indicators could be around five, and certainly fewer than 10. This headline list of indicators could be useful to the United Nations, the Paris Agreement, and national policymakers, as well as to the general public. The ideal list would capture the major outlines of climate change without either over-simplifying, or over-complicating, its characteristics.
This article draws on the WMO community’s ongoing discussions and does not present any final conclusions. As many institutions within the community are involved in producing indicators, any proposed set of indicators would eventually need to appear in a peer-reviewed scientific paper whose co-authors include the key WMO constituencies.
The makings of a headline indicator
WMO and its partners focus on the indicators of physical climate, for which they have the mandate and expertise, spanning the energy, water and carbon cycles. Socio-economic indicators of how climate impacts sectors such as health and agriculture are also, of course, critically important. Developing these indicators is a major challenge because of the diversity of climate impacts and a lack of systematically collected data on climate impacts in affected sectors from authoritative sources. This challenge is best addressed by other organizations and expert communities. WMO recognizes the need to obtain the broadest possible picture of climate change and therefore collaborates with these other organizations to provide information on weather, water and climate that is relevant to their work.
Most people are aware that the temperature – or more specifically, the global average temperature of the atmosphere just above the Earth’s surface – is rising, but this is not sufficient as an indicator of climate change. People focus on the surface-level atmosphere because that is where we live, and its temperature, which has been reliably measured for over 150 years, shapes our daily lives. But more than 90% of the excess heat trapped by humanity’s greenhouse gas emissions is stored in the ocean, with much smaller amounts absorbed by the atmosphere, the cryosphere and land. Therefore, the atmosphere’s temperature does not provide a complete picture of the Earth’s climate or of the full dimensions of climate change, and at worst can contribute to a false sense of security.
There are five criteria for the indicators on the short list:
- Relevance: each headline indicator should be a clear, understandable indicator of global climate change, with broad relevance for a range of audiences. Some such global indicators may also have value at the national and regional levels.
- Representativeness: the indicators as a package should provide a representative picture of changes to the Earth system related to climate change.
- Traceability: each indicator should be calculated using an internationally agreed (and published) method and accessible and verifiable data.
- Timeliness: each indicator should be calculated regularly (at least annually), with a short lag between the end of the period and publication of the data.
- Data adequacy: the available data needed for the indicator must be sufficiently robust, reliable and valid.
A key challenge in producing a set of headline indicators that are mutually supportive and consistent, and that form a truly coherent package, is the need to harmonize reference periods and baselines. The Paris Agreement uses the pre-industrial period as the reference period for global temperature, and indeed this provides a useful measure of humanity’s modern impact on the climate system. Scientists currently use several differing definitions of “pre-industrial period."
A number of periods have been proposed with respect to temperature observations, such as 1720–1800 (when industrialization truly got started) and 1850–1900 or 1880–1910 (based on the availability of instrument records). The available data indicate that these periods all give outcomes within 0.1 °C of each other.
In the case of greenhouse gases, for which ice cores provide reliable data well before the start of the instrumental period, the date 1750 is often used as the end of the pre-industrial period. For other indicators, such as precipitation, data from the pre-industrial period are not available. Only the use of a reference period that is based on data post-1980 would allow for the consistent handling of data sets based on satellite retrievals and reanalyses.
A closer look at six candidates
With the above criteria and constraints in mind, it can be quickly determined that some indicators are easy to support with credible measurements and are not too difficult to communicate. Temperature is a good example. Others are much more difficult. Even something as seemingly basic to climate as precipitation cannot easily be reduced to a single global indicator.
1. Global annual average surface temperature
As noted above, the Paris Agreement is best known for emphasizing the need to keep the global surface temperature to well below 2 °C above pre-industrial levels. The pre-industrial period still needs to be defined more clearly.
Of the three instrument-based global temperature records dating back to that time, the one maintained by the Hadley Centre/Met Office in the United Kingdom extends back to 1850. Those maintained by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) in the United States of America extend back to around 1880. The Japan Meteorological Agency maintains a global temperature data set back to 1891. The European Centre for Medium-range Weather Forecasting (ECMWF)/Copernicus provides reanalyses of these data sets. The China Meteorological Administration maintains a global land temperature data set and will, in time, likely provide a global land-and-sea temperature data set. Measurements from before the mid-1800s are based on proxy records such as tree rings and ice cores. A practical baseline based on the instrumental record, then, could be the 30-year period of 1880–1910.
The global surface temperature offers a relatively easy-to-understand indicator, but it reflects only part of the increases in energy in the global system. Nevertheless, as surface temperature is a critical target for the Paris Agreement, it would seem essential to include it in the set of climate headline indicators. It should be supplemented with temperature indicators keyed to the regional and national levels as well.
Scientists recognize that a better measure of the increase in the energy of the Earth system may be, for example, the top-of-atmosphere energy balance. But, recalling that indicators should be easily understood by policymakers and the general public, it is important to also recognize that this indicator would be more difficult for many non-scientists to interpret.
2. Ocean heat content
Well over 90% of the additional energy captured by human-made climate change is stored in the oceans. The remainder warms the land and melts ice, with only 1-2% directly warming the atmosphere. Thus, the increase in ocean heat is a good indicator of the warming of the Earth system as a whole.
The global measurements from the international network of Argo buoys make it possible to reconstruct a time series of the heat content in the top 2 000 metres (m) of the ocean from 1970 onwards. Reconstructed gridded ocean-heat content products are available from Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), NOAA and other organizations. The possibility of expressing these data as an average temperature should be explored.
A subsidiary indicator could be the heat content down to only 700 m, since this would make a longer time series, is possible.
3. Atmospheric concentrations of carbon dioxide
Because the level of greenhouse gases in the atmosphere influences how much energy is trapped in the Earth system, atmospheric concentrations provide a useful indicator of climate change. Currently, the WMO Global Atmosphere Watch (GAW) gathers data from around the world on the concentrations of major greenhouse gases and publishes an annual report with a time lag of ten months.
Traditionally, the UNFCCC has used an indicator of concentrations that combines all greenhouse gases – carbon dioxide, methane, nitrous oxide, HCFCs, etc. – into one basket. Each gas is translated into a CO2-equivalent to produce a single number using Global Warming Potentials (GWPs) with a 100-year time horizon. However, the use of GWPs is now questioned by some Parties to the Convention given the differing timescales of the impacts of the different gases: methane has a large immediate impact but this diminishes over decades while CO2 will have impacts for centuries. Because carbon dioxide is currently the most important greenhouse gas directly emitted by humanity and is the target of many emissions-reduction efforts, experts believe it may be more transparent to focus on annual trends in atmospheric concentrations of CO2 as the indicator. Subsidiary indicators could include concentrations of methane and of nitrous oxide.
GHG emissions increase concentrations, while sinks reduce concentrations by removing gases from the atmosphere and storing them in the ocean, biosphere or geosphere. To better understand changes in concentrations, as well as sources and sinks, WMO is working with partners to explore how atmospheric monitoring instruments can measure concentrations and local CO2 sinks and sources through an Integrated Greenhouse Gas Information System (IG3IS).
4. Global mean sea level
As described above, the ocean plays a vital role in shaping our climate by storing vast quantities of heat and moving it around the globe. How climate change affects the ocean also has great significance for humanity. Climate impacts include warming waters and acidification, which affect fish stocks and other biodiversity, and sea-level rise with its implications for coastal cities and communities.
Sea-level rise is routinely assessed by CSIRO, the University of Colorado Sea Level Research Group, and others. Annual analyses of sea level can take a few months before they are released. Thanks to improved observations from satellites and Argo ocean profiling floats, scientists are able to estimate the various sources – melting ice, thermal expansion of ocean water and changes to water storage on land – that contribute to global seal-level changes. More recently, satellites have also started to measure changes in the mass of water in the oceans, bringing an independent check on measurements of the changes in these contributions. However, these estimates have been available only in recent years, and in some cases with a considerable time lag. Moreover, as changes in sea level are non-uniform across the world, global sea-level information should be supplemented by regional analyses where these are relevant.
5. Changing extent or mass of the cryosphere
The Earth’s cryosphere includes solid precipitation, snow cover, sea ice, lake and river ice, glaciers, ice caps, ice sheets, permafrost and seasonally frozen ground. The cryosphere provides some of the most useful indicators of climate change, yet is one the most under-sampled domains of the Earth system. The most readily produced data and analyses of the cryosphere include sea-ice extent in the Arctic and Antarctic (updated daily and monthly by the US-based National Snow and Ice Data Center), Northern Hemisphere snow cover (the Rutgers University Snow Lab) and Mountain glacier analysis (the Swiss-based World Glacier Monitoring Service). Measures of southern hemisphere snow cover are not as reliable. Analyses of changes in the Greenland ice sheet are made available by the Danish Meteorological Institute, but the record is short.
Choosing a single indicator for the cryosphere may not be possible. Arctic and Antarctic ice should be reported as separate numbers as the Arctic and Antarctic ice behave very differently. Over land, the behaviour of the cryosphere involves complex interactions at regional and local scales. This makes it difficult to develop indicators that depict the global impacts of climate on the cryosphere in a coherent manner.
Currently, the best approach may be to adopt a three-part cryosphere indicator: Arctic sea-ice extent, Antarctic sea-ice extent and Northern Hemisphere snow cover. Analyses of these various facets of the cryosphere would help draw the attention of policymakers to the most sensitive parts of the cryosphere. Simple and easily understood indicators would be needed as well as scientific explanations of where scientific knowledge of the cryosphere is still not robust.
6. Global precipitation
The WMO Global Precipitation Climatology Centre (GPCC), operated by Deutscher Wetterdienst, provides the most comprehensive data available. GPCC uses data collected through WMO systems and channels. Currently, the precipitation analyses are provided on a monthly basis after they have been quality controlled. Information on the geographical distribution of precipitation extremes on seasonal, annual and multi-year time scales can provide good insights into shifts in the patterns and nature (solid, liquid) of global precipitation, changes in geographical distribution, trends in drought and intense rain storms, and the influence of other changing features of the climate system, such as monsoons, the El Niño-Southern Oscillation, dipoles, and so forth. These insights can be used to inform decisions on adaptation.
As a single indicator, however, the global precipitation anomaly would not make a good climate indicator. Climate projections suggest an intensification of the global hydrological cycle, but the uncertainties and variability at the local level are large. It is therefore unlikely to provide a clear signal. A better approach would be to have indicators based on an analysis of the global distribution of precipitation that captures annual, multi-year and long-term variations.
Developing indicators for extreme events
Change in the frequency and/or intensity of extreme weather and climate events resulting from climate variability and change is one of the most important impacts of climate change. In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCCC) highlights eight key risks for human well-being that will require adaptation measures. Most of these risks are driven by extreme events, including storm surges, coastal and inland flooding, extreme heat, drought, flooding, and precipitation variability and extremes. As noted below, this is echoed in the Intended Nationally Determined Contributions (INDCs) delivered by Parties to the UNFCCC, which identify flooding, sea level rise and drought or desertification as the main causes of concern.
Since what is perceived as an extreme event varies from place to place, indicators could be based on the frequency of deviations from a typical range, such as the number of times a specific percentile is exceeded in a specific place.
Consistent and meaningful indicators for extreme precipitation, droughts, heatwaves and other hazards have been proposed that could reflect the influence of climate change on extreme events at the global scale. The frequency of intense tropical cyclones has been discussed as an indicator. However, the various WMO Regional Specialized Meteorological Centres (RSMCs) have different ways of categorizing these storms according to their intensity. There are also inconsistencies in the coverage of historical time series, so it is often difficult to get useful information that can be put into historical or geographical context. A global index of Accumulated Cyclone Energy (ACE) as an indicator for cyclonic activity is being provided by NOAA/NCEI. It will be explored in the context of WMO Statement on the State of the Global Climate in 2017 and, if successful, it will be considered as a global indicator for tropical cyclones. However, how this indicator may relate to climate change is unclear at this stage.
In 2015, the World Meteorological Congress decided “to standardize weather, water, climate, space weather and other related environmental hazard and risk information and develop identifiers for cataloguing weather, water and climate extreme events.” This decision will promote the increasingly standardized compilation of data on hydro-meteorological events – the main cause of losses and damage – by national governments.
As this brief survey makes clear, several indices – global temperature, CO2 concentrations, ocean warming, polar sea-ice extent and sea-level rise – seem to be robust and obvious choices for the set of headline climate science indicators. For others, it is more challenging to identify indices that satisfy the criteria listed above while also being clear and easy to communicate to policymakers and the general public.
Looking ahead, it is important to develop further indicators that support global action on climate change. Key instruments for enabling climate action under the UNFCCC’s Paris Agreement are Parties' National Determined Contributions (NDCs), which will update and refine the existing INDCs. It is vital that these national climate actions benefit from scientific information on climate variability, trends and extremes. As of October 2017, 162 INDCs had been submitted, covering 190 Parties, representing 96% of the Parties to the Convention. Of these, 137 Parties also included an adaptation component where Parties reported their vulnerabilities. In terms of climate hazards, the main sources of concern identified by most Parties are flooding, sea-level rise and drought or desertification. While sea level is an indicator, this highlights the need for the indicators of extreme precipitation events discussed in the previous section to address these issues.
Priority areas and sectors identified in the adaptation component of the INDCs include water, agriculture, health, ecosystems, infrastructure, forestry, energy, disaster risk reduction, food security, coastal protection and fisheries. These areas include all priority areas of the Global Framework for Climate Services (GFCS). Thus, additional indicators that contribute to impacts across these sectors are also needed.
In the intermediate term, WMO can make available information on most of the indicators discussed in this article through its publications, such as the Statement on the State of the Global Climate and the Greenhouse Gas Bulletin. These annual reports will be harmonized with the IPCC Assessment Reports, however, to ensure that WMO’s annual reporting and the more comprehensive, but less frequent, IPCC reports tell a consistent story.
Further work is also needed to refine the indicators explored above. Consensus could be achieved through the publication of a peer-reviewed article involving interested authors, many of whom contribute to the WMO Technical Commissions, GAW, GCOS, WCRP and the IPCC Working Groups.
Finally, the WMO community needs to continue engaging strongly with organizations that can provide indicators of key climate impacts, including through GCOS and the GFCS.
With thanks to the following contributors from the WMO Secretariat: Maxx Dilley, Omar Baddour and Amir Delju as well as Carolin Richter from GCOS.