BLOG | What does a 1.5⁰C increase in global temperature mean for deltas?

By Robert Nicholls, Principal Investigator for DECCMA and Professor of Coastal Engineering at the University of Southampton.


Deltas are a climate change hotspot, where the effects of climate change coincide with large numbers of people. Sea level rise is a major threat to deltas, bringing risks of flooding and erosion. As the world tries to limit the global temperature increase to 1.5⁰C, the DEltas, vulnerability and Climate Change: Migration and Adaptation (DECCMA) project has been looking at how deltas will be affected by increases in temperature of 1.5⁰C, 2⁰C or 3⁰C.

Following the historic Paris Agreement, 1.5 ⁰C has become a hot topic.  The Paris Agreement commits developed and developing countries to global temperature increase to 2⁰C, with the aspiration to limit to 1.5⁰C.  These numbers are widely believed to be critical thresholds beyond which significant changes in the natural environment would be experienced (known as planetary boundaries).

The issue of 1.5⁰C has recently been high on the political agenda again, as the 23rd Conference of the Parties to the UNFCCC met in Bonn to discuss a framework for reporting climate action to monitor the commitments made under the agreement.  Knowing the implications of a 1.5⁰C increase informs the “ambition mechanism”, whereby stocktakes of progress are due to be taken every 5 years, with a view to then revising and updating mitigation and adaptation commitments.  Improvements in science play a key input to ensuring that these commitments remain ambitious and on target to limit the damaging effects of climate change.

Deltas are home to 500 million people worldwide, as well as being natural environments that generate livelihoods, income and essential ecosystem services.  DECCMA has been investigating the effects of climate change in four study sites across three deltas across Africa and Asia: the Ganges-Brahmaputra-Meghna (GBM) megadelta in Bangladesh and the Indian Bengal component in India; and the smaller deltas of Mahanadi in India and Volta in Ghana.

Given the interest in 1.5⁰C, we have used our customised integrated assessment model –the Delta Dynamic Integrated Emulator Model – to look at the likely changes in flooding (in terms of depth of flood and area affected) and the impacts on population in the GBM in Bangladesh under three different scales of temperature increase: 1.5°C, 2.0°C and 3.0°C.

If we continue with relatively high rates of greenhouse gas emissions, models show that a 1.5°C increase could occur from 2011 to 2033.  Rates of temperature increase have already been significant and rapid.  Observed changes in temperature over the 20th century showed an increase in 0.7⁰C.  In comparison, in the readjustment period since the last ice age global temperatures have only increased by between 4-7c over 5000 years.

Sea level rise of 5-14cm is associated with an increase in global temperature of 1.5⁰C.  This may not seem a lot, and it is particularly difficult to find a reference period because sea levels have varied significantly over the last 20,000 years, reflecting glacial periods and the readjustment of land masses.  But, as an indication, sea levels rose by less than 2mm over the 20th century, so the projected increase is over 20 times more than that.

Until 2040 the differences that are likely from a 1.5⁰C increase and a 2⁰C increase are indistinguishable largely due to the year on year variability that is already characteristic of deltas.

If the temperature increase reaches 3⁰C, some of consequences more than double.  The area flooded under 3⁰C is more than 2.5 times that under 1.5⁰C of such sea level rise, for example.  Those at greatest risk are in the central regions and northeast, where there are fewer polders to protect the land.

The good news is that there is still time to implement adaptation – if we act now.  Our team has investigated adaptation and found 93 documented examples in our study deltas spanning agriculture, water management and disaster risk reduction.  We are now in the process of developing an integrated assessment model that will give us insights into adaptation needs and options under various future scenarios.


For more information, see:

Brown Sally, Nicholls Robert J, Lázár Attila, Hazra Sugata, Appeaning Addo Kwasi, Hornby Duncan D, Hill Chris, Haque Anisul, Caesar John and Tompkins Emma, What are the implications of sea-level rise for a 1.5°C, 2°C and 3°C rise in global mean temperatures in vulnerable deltas? Submitted to Regional Environmental Change.