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Top 10 Climate Change Indicators : Reliable Yardsticks

Climate Change indicators are observations that can be used to track the current state of climate and its trends.

Climate is defined as the average weather conditions prevailing at a place or a region over a long period. At a broader scale climate systems include land, atmosphere, ocean and the cryosphere. The World Meteorological Organisation (WMO) specifies a period of 30 years to define the climate of a place.

Climate is not static. It exhibits both short term variability as well as changes over a long period of time. Variation in climate from one year to another is known as natural variability. Drought in one year and floods in another is a part of natural variability. Changes in weather elements and the climate system observed over a long period of time is called climate change.

The earth’s climate has changed throughout history. In the last 650,000 years there have been cycles of glacial advance and retreat, with an abrupt end of the last ice age about 11,000 years ago. It marked the beginning of the modern climate era and of human civilization. Most of these climate changes are attributed to very small variations in earth’s orbit that change the amount of solar energy received by the Earth. These climate changes used to take place over a very long period spread over thousands of years. The current warming trend is of significance because most of it is caused by human activity since the mid-20th century and proceeding at a rate unprecedented.

Climate Change Indicators

Climate Change indicators are observations that can be used to track the current state of climate and its trends. In order to bring objectivity in monitoring climate change, scientists have developed climate change indicators. The indicators are quantified and objective, based on data collected by countries all over the world. An indicator is selected based on its relevance, representativeness, traceability, timeliness and data adequacy.

Climate change indicators may be physical, ecological, or societal. These help in monitoring how climatic conditions are changing and to assess risk and vulnerabilities. Depending on areas of application, number and type of indicators can vary from as many as 40 indicators monitored by the United States Environment Agency (USEPA). These cover greenhouse gases, weather and climate, oceans, snow and ice, health and safety and ecosystems (USEPA 2016).
The primary focus of meteorologists is to observe and predict physical components of the climate system. The WMO has identified six primary indicators (WMO 2017):

    1. Global annual average surface temperature
    2. Atmospheric concentrations of carbon dioxide
    3. Ocean heat content
    4. Global mean sea level
    5. Mass of the cryosphere or Global extent
    6. Global precipitation

The Agenda 2030, adopted by the United Nations in 2015, seeks to use indicators to track progress on the Sustainable Development Goals, including SDG13 on combating 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.

1. Global temperatures as a climate change indicator

The year 2019 was the second warmest year in the 140 year record, with a global land and ocean surface temperature departure from average of +0.95°C (NOAA 2019). This value is only 0.04°C less than the record high value of +0.99°C set in 2016 and 0.02°C higher than the now third highest value set in 2015 (+0.93°C ).

The five warmest years in the 1880–2019 record have all occurred since 2015, while nine of the 10 warmest years have occurred since 2005. The year 1998 currently ranks as the 10 warmest, the only year before 2005 that made it to the record.

After that, the year 2019 marks the 43rd consecutive year (since 1977) with global land and ocean temperatures, at least nominally, above the 20th century average indicating the slow but steady rise of a changing climate. This year (2019) began in a weak-to-moderate El Niño, transitioning to El Niño–Southern Oscillation (ENSO)-neutral conditions by July. During the year, each monthly temperature ranked among the five warmest for their respective months on record, with the months of June and July recording warm.

The global annual temperature has increased at an average rate of 0.07°C per decade since 1880 and over twice that rate (+0.18°C) since 1981; surface air temperature anomaly for the year 2019 is shown in Fig 1.

Fig 1 : Surface air temperature anomaly for 2019 with respect to the average 1981-2010 ( Source : European Centre for Medium Range Weather Forecast ERA5 data)

2. Atmospheric concentration of greenhouse gases: A tell-tale climate change indicator

Increasing levels of greenhouse gases in the atmosphere are a major driver of climate change. Atmospheric concentrations reflect a balance between sources (including emissions) and sinks. Global carbon dioxide (CO2) concentrations reflect the balance between emissions due to human activities and uptake by the biosphere and ocean.
The WMO report on greenhouse concentration shows that 2015-2019 has seen continued increase in CO2 and other greenhouse gases in the atmosphere (Fig 2). CO2 growth rates are nearly 20 per cent higher than the previous five years. CO2 remains in the atmosphere for centuries and in the ocean for an even longer period.

In 2018, greenhouse gas mole fractions reached new highs, with globally averaged mole fractions of CO2 at 407.8±0.1 ppm, CH4 at 1869±2 ppb and N2O at 331.1±0.1 ppb (Fig. 2). The annual increases in the three main greenhouse gases were larger than the increases in the previous year and the 10-year averaged growth rates. The global averaged mole fractions in 2018 constitute, respectively, 147, 259 and 123 per cent of pre-industrial (1750) levels. Global average figures for 2019 will not be available until late 2020, but real-time data from specific locations, including Mauna Loa (Hawaii) and Cape Grim (Tasmania) indicate that levels of CO2, CH4 and N2O continued to increase in 2019. Latest March 2020 CO2 observation reported by Mauna Loa Observatory is 414.5 ppm.

Figure 2: Globally averaged concentration of CO2 from 1984 to 2018

3. Sea level rise:

Sea level rise is one of the less visible climate change indicators but a very reliable one. Over the five-year period May 2014 -2019, the rate of global mean sea-level rise has amounted to 5 mm per year, compared with 4 mm per year in the 2007-2016 ten-year period. This is substantially faster than the average rate since 1993 of 3.2 mm/year. The contribution of land ice melt from the world glaciers and the ice sheets has increased over time and now dominate the sea level budget, rather than thermal expansion.

In 2019, the sea level continued to rise (Fig 3), with the global mean sea level reaching its highest value since the beginning of the high-precision altimetry record (January 1993). The average rate of rise is estimated at 3.24 ± 0.3 mm yr–1 over the 27 year period, but the rate has increased over that time. A greater loss of ice mass from the ice sheets is the main cause of the accelerated rise in the global mean sea level on top of steady increases from the expansion of ocean waters driven by warming due to climate change.

Figure 3: Global mean sea level evolution from January 2016 to 2019 from high precision altimetry.

4. Ocean Heat Content

The ocean is an important part of the Earth system. The rate of change in ocean heat content is a measure of global warming and a robust climate change indicator, as it represents a large proportion of the heat accumulating in the climate system.

Thermal expansion from ocean warming, combined with melting of ice on land, leads to sea level rise, which affects coastal areas. Changes in ocean chemistry associated with rising CO2 concentrations in the atmosphere are altering the pH of the oceans. Ocean heat content (OHC) is a fundamental metric for climate change as it is a measure of heat accumulation in the earth system.

Human-induced atmospheric composition changes cause a radiative imbalance at the top of the atmosphere – earth’s energy imbalance – which is driving global warming and climate change. Due to the ocean’s large heat capacity, the majority (~90 per cent) of this accumulated heat is stored in the global ocean. Consequently, the ocean is warming, with wide-reaching impacts on the earth climate system.

For example, OHC increase contributes more than 30 percent of observed global mean sea-level rise through thermal expansion of sea water. Ocean warming is altering ocean currents, and indirectly altering the storms.

The year 2018 had the largest ocean heat content values on record measured over the upper 700 meters, with 2017 ranking second and 2015 third (Fig 4). The ecological costs to the ocean, however, are high, as the absorbed CO2 reacts with seawater and changes the acidity of the ocean. There has been an overall increase in acidity of 26 per cent since the beginning of the industrial revolution.

Fig 4: Ocean Heat Potential

5. Mass of the cryosphere or global extent: Inversely related to climate change

Throughout 2015-2018, the Arctic’s average September minimum (summer) sea-ice extent was well below the 1981-2010 average, as was the average winter sea-ice extent (Fig 5).

The four lowest records for winter occurred during this period. Multi-year ice has almost disappeared. Antarctic’s February minimum (summer) and September maximum (winter) sea-ice extent values have become well below the 1981-2010 average since 2016. This is in contrast to the previous 2011-2015 period and the long term 1979-2018 period.

Antarctic summer sea ice reached its lowest and second lowest extent on record in 2017 and 2018, respectively, with 2017 also being the second lowest winter extent. The amount of ice lost annually from the Antarctic ice sheet increased at least six-fold, from 40 Gt per year in 1979-1990 to 252 Gt per year in 2009-2017.

The Greenland ice sheet has witnessed a considerable acceleration in ice loss since the turn of the millennium. The World Glacier Monitoring Service (WGMS) reference glaciers indicates an average specific mass change of −908 mm water equivalent per year during 2015-18, higher than in all other five-year periods since 1950. These are glaring climate change indicators that show the rapidity with which changes are taking place.

Fig 5: Monthly September and March Arctic sea-ice extent anomalies (relative to the 1981–2010 average) for 1979– 2019
Sources: US National Snow and Ice Data Center (NSIDC) and EUMETSAT Ocean and Sea Ice Satellite Application Facility.

6. Global Precipitation Patterns as a Climate Change Indicator

Unusually dry conditions in relation to long-term means for 2019 were observed in Australia and western Indonesia and surrounding countries. Also, southern Africa, Central America and parts of South America received abnormally low precipitation amounts. Large areas with unusually high precipitation amounts were observed in the Central United States, Northern Canada, northern Russia, South-west Asia, northern China and eastern Africa.

There was a large precipitation deficit in India in June as the onset of the monsoon was delayed. However, associated with the positive IOD phase, the withdrawal of the Indian monsoon was also delayed and there was an excess of precipitation in the following months for all regions except north-east India. In addition, positive precipitation anomalies in eastern Africa and negative anomalies in the Malay Archipelago and Australia are associated with the positive IOD phase, which prevailed through most of the second half of 2019. This unpredictable change in precipitation levels is yet another indication of climate change.

7. Extreme events and Climate Change

Hydro-meteorological hazards account for more than 90 per cent of the natural disasters. The dominant disasters are storms and flooding, which have also led to highest economic losses. Heatwaves and drought have led to human losses, intensification of forest fires and loss of harvest. These extreme events are climate change indicators that exhibit the stress under which ecosystems lie.

8. Heat Waves : Hallmark climate change indicator

Heat waves, which were the deadliest meteorological hazard in the 2015-2019 period, affecting all continents and resulting in numerous new temperature records. Almost every study of a significant heat wave since 2015 has found it to be the hallmark of climate change. The year 2019 also saw numerous major heat waves.

Amongst the most significant were two heat waves that occurred in Europe in late June and late July (Fig 16). The first one reached its maximum intensity in southern France, where a national record of 46.0 °C (1.9 °C above the previous record) was set on 28 June at Vérargues (Hérault). Japan experienced two heat waves that were notable in different ways.

The first occurred in late May, with unusually high temperatures, including 39.5 °C (the equal highest on record for any time of year on the island of Hokkaido), but limited impacts.

The second, in July, was less unusual in a meteorological sense but had much greater health impacts as it occurred during the peak of summer and was focused in the more heavily populated area of Honshu. Australia had an exceptionally hot summer in 2018–2019. The mean summer temperature was the highest on record by almost 1 °C, and January was Australia’s hottest month on record.

Most of the country was affected. The occurrences of heat waves in hitherto unaffected regions are deeply concerning and once again global warming due to climate change is to be blamed.

9. Tropical Cyclones: A significant climate change indicator

The largest economic losses were associated with tropical cyclones. The 2017 Atlantic hurricane season was one of the most devastating on record with more than 125 billion USD in losses associated with Hurricane Harvey alone. On the Indian Ocean, in March and April 2019, unprecedented and devastating back-to-back tropical cyclones hit Mozambique. Global tropical cyclone activity in 2019 was above average.

The northern hemisphere had 72 tropical cyclones, compared with the average of 59. The 2018–2019 southern hemisphere season was also above average, with 27 cyclones, the highest number in a season since 2008–2009. It was a particularly extreme cyclone season in the North Indian Ocean.

Three cyclones reached maximum sustained winds of 100 kn or more, the first known instance in a single season, and the seasonal ACE was the highest on record by a large margin. Fani was the most significant cyclone to affect India since 2013, making landfall in the east, on the Odisha coast, on May 3, with sustained winds of 100 kn, having earlier peaked at 135 kn in the Bay of Bengal.

There was significant damage in coastal areas and loss of life, although extensive evacuations in affected coastal areas greatly reduced the human impact. Kyarr, in October, was one of the strongest cyclones on record in the Arabian Sea, but did not make landfall, although associated high seas and storm surges affected some coastal areas.

10. Wildfires: Major influencer as a climate change indicator

Wildfires are strongly influenced by weather and climate phenomena with global warming due to climate change being one of the major influencers. Drought substantially increases the risk of wildfire in most forest regions, with a particularly strong influence on long-lived fires. The three largest economic losses on record from wildfires have all occurred in the last four years.
In many cases, fires have led to massive releases of carbon dioxide to the atmosphere. Summer 2019 saw unprecedented wildfires in the Arctic region. In June alone, these fires emitted 50 megatons (Mt) of carbon dioxide into the atmosphere. This is more than that released by Arctic fires in the same month from 2010 to 2018 put together. There were also massive forest fires in Canada and Sweden in 2018. Also significant were widespread fires in the tropical rain forests in Southern Asia and Amazon, which have had impacts on the global carbon budget. The Australian bushfire of 2019-20 occurred in the summer of 2019, Australia’s hottest on record thereby linking the event with climate change.

Way forward

Ongoing trend in the rise of global average temperature due to climate change continued in 2019, with a global average temperature of 1.1oC above estimated pre-industrial (1850-1900) averages. The year 2019 was also the second warmest year. In 2018-19, greenhouse gases concentration reached new highs. Latest March 2020 observation of CO2 reported by Mauna Loa Observatory is 414.5 ppm. Increasing levels of greenhouse gases in the atmosphere are a major driver of climate change.

Along with rise in temperature and increase in greenhouse gases, other climate change indicators also confirm the ongoing trend of climate change. Sea levels are rising at an increasing pace through greater warming of oceans both at the surface and in the depths. Enhanced melting of Greenland’s ice and of glaciers also contributing to sea level rise. It is exposing coastal areas and islands to flooding and the submersion of low-lying areas. The year witnessed severe Heat Waves in Europe, Japan and Australia. Heat waves combined with prolonged drought led to wildfires of unprecedented size in Australia, Siberia and Arctic regions. In line with global trends, India too experienced above average temperatures, rainfall, cyclonic storms in 2019. These climate change indicators are nature’s way of indicating that our planet is under stress and needs immediate remedial action to prevent a global catastrophe.


1. United States Environmental Protection Agency (US EPA). 2016. Climate change Indicators in the United States 2016: US EPA, Washington D.C.: United States. Available at:
2. World Meteorological Organization (WMO). 2017. Weather ready, Climate smart – Supporting the 2030 Agenda for Sustainable Development: WMO, Geneva: Switzerland. Available at:
3. National Centers for Environmental Information (NCEI). 2019. State of the Climate – Global Climate Report for Annual 2019: NCEI, North Carolina: United States. Available at:

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