This month, as part of our Get ClimateReady campaign, we will be exploring the theme of sea level rise, a topic which has been of significant interest this year. Simply put, “sea level rise” refers to the rise in the ocean surface as a result of climate change. Rising sea levels can cause significant impacts for infrastructure, coastal communities and wildlife across the globe, and it is clear that a combined approach, implementing both mitigation and adaptation measures, is necessary to avoid the worst of these impacts. Understanding the changes in sea level – what causes them, why, and how quickly they occur, is imperative if we are to adopt the most effective solutions.
There are two main aspects of sea level rise that are of particular interest to climate scientists:
Global and regional mean sea level
Increases in global mean sea level are caused by two main processes; thermal expansion of oceans as they increase in temperature due to global warming, and the addition of more water to the oceans – mostly from melting ice sheets and glaciers.
Whilst the total increase in ocean volume relates to global mean sea level rise, regional sea level can be influenced by additional factors such as changes in ocean currents, the distant effects of land ice melt and the rise and fall of land masses. These factors affect sea level rise around the UK. For example, while the land is rebounding in Scotland after the last ice age, in southern England it is sinking, which affects the sea level rise experienced in these regions.
Similarly, gravitational effects can influence sea level at a regional scale. The large amount of ice in the Greenland and Antarctic ice sheets means that water is pulled towards them by gravity. As this ice melts, the gravitational attraction decreases, which leads to a drop in sea level near the ice sheet and a rise much further away. Areas that are in between the areas of fall and rise experience very little sea level rise from the ice sheet melt. Due to its geographic location, melting of the Greenland ice sheet will have a much lesser effect of UK sea level rise than melting of the Antarctic ice sheet.
In this article from the Guardian, Met Office Climate Scientist Dr Matt Palmer explains more about the gravitational effects which influence sea levels.
Extreme sea level
Extreme sea level typically refers to the maximum sea level that is experienced during a storm. Storm surges can contribute to significant extremes in sea level, occurring when sea levels rise due to a combination of a reduction in atmospheric pressure, water being forced onto the coast by the wind, and in some cases funnelling by the local bathymetry (variations in the ocean floor).
Climate change and sea level
As the planet warms, so too do our oceans, directly contributing to thermal expansion and the melting of ice sheets and glaciers, which lead to increases in global and regional mean sea level rise. The slow response of the oceans and ice sheets to climate change mean that sea level will continue to rise for centuries, even under scenarios where future temperature rise is stabilised/stopped.
The Intergovernmental Panel on Climate Change (IPCC) is the United Nation’s body for assessing the science related to climate change. In August 2021, the IPCC published the Working Group 1 (WG1) contribution to the Sixth Assessment Report (AR6), a peer-reviewed publication which examines the physical science of climate change, based on contributions from thousands of international scientists. The report confirmed that global mean sea level has increased by 20cm since the early 20th century and that human influence was the main driver of these increases since at least the 1970s. Extreme sea level events are also expected to become more frequent in the future and IPCC assessed that, compared to the recent past, extreme sea levels will occur about 20 to 30 times more frequently by the year 2050.
Dr Helene Hewitt, of the Met Office, was a co-ordinating lead author on the chapter covering Oceans, Cryosphere and Sea Level. In an article exploring the findings of the IPCC’s report, Helene took a closer look at what we know about past and future sea level rise, and examined projections for the future, based on different climate scenarios.
Speaking about the report, she said:
“This report demonstrates that oceans are continuing to warm, ice is melting and sea level is rising. Many of these changes will not stop immediately if we reduce emissions but they can be slowed down and crucially, we will limit the risk of rapid ice loss from Antarctica which otherwise could lead to additional metres of sea level rise over the coming centuries”.
More recent climate reports provide further clear evidence to support the fact that climate change is influencing sea levels, both around the UK and across the globe. Published in May, the World Meteorological Organisation’s State of Climate report found that global mean sea level reached a new record high in 2021. The report confirmed the IPCC’s findings that the rate of sea level rise had increased, estimating that sea level has risen at an average rate of 4.5mm per year over the period 2013 – 2021, more than double of that between 1993 – 2002. Such an increase can be attributed to the accelerated loss of ice mass from ice sheets.
The increasing rate of sea level rise seen in the global mean is also apparent in regional sea level around UK. The latest State of the UK Climate report for 2021 by the Met Office, found that the rate of sea level rise around the UK is increasing, with selected locations seeing a sea level increase of 3.0-5.2mm per year, compared to around 1.5mm per year in the 1900s.
The National Oceanography Centre (NOC) provided the input for the sea level section of the State of the UK Climate report.
Dr Svetlana Jevrejeva, a Principal Research Scientist at the NOC, said:
“Our long-term records show that over the past few decades, rate of sea level rise in the UK is increasing. As sea levels rise there can be greater impacts from storm surges. Last year storm surges of over 1.5 m were seen during Storm Arwen, but extreme sea levels were avoided as this occurred during low water and a neap tide”.
Throughout August we will be continuing to explore the topic of sea level rise and its wider impacts. In our next blog, we’ll be taking a closer look at sea level rise – how it is monitored, the impacts it can cause, and the different approaches that can be taken to mitigate these, both here in the UK and from a global perspective. Follow #GetClimateReady on Twitter as we explore the topic of ‘sea level rise’.
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