Why are Temperature or Precipitation Changes a Cause of Concern?

Temperature is an essential climate quantity that directly affects human and natural systems. The global mean surface temperature is a key indicator of climate change because it increases quasi-linearly with cumulative greenhouse gas emissions as documented in multiple assessment reports of the Intergovernmental Panel on Climate Change (IPCC) including the most recent Fifth Assessment Report that was released in 2013. 

According to the Assessment of Climate Change Impact in India Report released by the Ministry of Earth Sciences of the Government of India, a significant overall warming trend in all India averaged annual mean surface air temperature for the long-term period 1901-2010 was assessed using the estimates derived from the India Meteorological Department (IMD) gridded monthly station data. 

Greenhouse Gases (GHGs) act as a blanket to keep the temperature levels high - higher the GHGs, thicker the blanket. The surface air temperature, typically measured at 2 m

above the ground, varies from one region to another within India. This temperature also fluctuates naturally in interannual and decadal time scales in the background of human-induced changes in the climate. One of the important contributors of the observed changes in temperature not caused by human activities is the natural internal climate variability, which refers to the chaotic short-term fluctuations around the mean climate over a region or at a location.

The temperature projections are obtained by driving climate models with different future forcing scenarios. These projections include the response of the climate system to external forcing (e.g. changing greenhouse gas concentrations), internal variability and uncertainties associated with differences between models. The multi-model ensemble averages out the internal variability and model differences to a large extent and provides an estimate of the response of the climate system to forcing.

According to the Royal Society, even though an increase of a few degrees in global average temperature does not sound like much, global average temperature during the last ice age was only about 4 to 5 °C (7 to 9 °F) colder than now. Global warming of just a few degrees will be associated with widespread changes in regional and local temperature and precipitation as well as with increases in some types of extreme weather events. These and other changes (such as sea level rise and storm surge) will have serious impacts on human societies and the natural world.

In order to improve the assessment of India’s observed and projected warming and its impact, the following gaps need to be addressed: 

• The uneven spatial distribution of temperature observation sites over India lead to errors in the assessment of present-day changes in temperature. 

• More research is needed to understand whether the increased water vapour under conditions of regional warming is leading to significant positive feedback on human-induced climate change, as water vapour is the most important contributor to the natural greenhouse effect.

•  Assessment of joint projections of multiple variables over India are needed to understand the key processes relevant to future predicted significant increases in temperature variability and extremes, for example, projected changes by combining mean temperature and precipitation; linking soil moisture, precipitation and temperature mean and variability; combining temperature, humidity, etc.

As far as precipitation changes are concerned, there has been a decreasing trend in the all-India annual, as well as summer monsoon mean rainfall during 1951– 2015, notably so over areas in the Indo-Gangetic Plains and the Western Ghats. 

Increasing concentrations of anthropogenic aerosols over the northern hemisphere appear to have played a role in these changes. The frequency of localized heavy rain occurrences over India has increased during 1951– 2015. Urbanization and other land use, as well as aerosols, likely contribute to these localized heavy rainfall occurrences.

With continued global warming and expected reductions of aerosol concentrations in the future, climate models project an increase in the annual and summer monsoon mean rainfall, as well as frequency of heavy rain occurrences over most parts of India during the twenty-first century. The interannual variability of summer monsoon rainfall is projected to increase through the twenty-first century.

The intensity, length and timing of monsoon are related to atmospheric moisture content, land–sea temperature contrast, land surface feedback, atmospheric aerosol loading and other factors. Overall, monsoonal rainfall is projected to become more intense in future, and to affect larger areas mainly due to increase in atmospheric moisture content with temperature. The temperature gradient between land and sea, regional distribution of land and ocean as well as topography play a major role in monsoon.

Global as well as regional models project an increase in seasonal mean rainfall over India while also projecting a weakening monsoon circulation. However, this weakening of circulation is compensated by increased atmospheric moisture content leading to more precipitation. Frequency of extreme precipitation events may increase all over India, and more prominently so over the central and southern parts as a response to enhanced warming. Monsoon onset dates are likely to be early or not to change much, and the monsoon retreat dates are likely to be delayed, resulting in lengthening of the monsoon season.

Know more:

This story is part of a four part series on the Climate Change Impact Assessment Report by the Ministry of Earth Sciences.

The other parts can be found here, here, here, and here.

Do more:

Have a look at this piece about the report, or reach out to us at submissions.scalesandstates@gmail.com for feedback, queries, and sharing your thoughts.