Today at the Woods Hole Oceanographic Institute, I am presenting on chapter 13 in the IPCC’s 5th version of its Assessment Report, called AR5. The IPCC has released similar reports since the late 80’s summarizing the most up-to-date science on climate change. The section that I am presenting on today comes from the Physical Science Working Group (Working Group 1), and it summarizes the latest scholarship on the physical science of sea level change (SLC).The other two Assessment Reports that the IPCC releases at regular intervals involve mitigation (actions that we can take to reduce greenhouse gas emissions) and adaptation (how we adjust global settlements, societies, and economies in accordance with the effects of climate change).
My presentation deals with one of the more-talked-about facets of climate change: sea level rise. Coincidentally, the contents of the National Climate Assessment made the front page of the NY Times today in terms of American cities at risk from SLC, mainly Miami. This blog entry summarizes some key points from my presentation, which in turn summarizes some key points from this 80 page technical document. If you want the whole presentation, it is filed under the Lecture Slides heading on this website (~60 minute presentation).
Primary contributing factors to Sea Level Change (SLC)
The primary driver of sea level rise is expansion of the ocean as it warms (thermal expansion). This is unfortunate, since observations show that the largest increase in the storage of heat in our climate system has been in the oceans. The other primary driver of SLC is the transfer of water as it is currently stored on land (particularly from land-ice i.e. glaciers and ice sheets).
Anticipated Sea Level Rise
IPCC Assessment Reports 1-4 have given estimates (ranges) of projected rise in sea level each year looking ahead to the next century. Below I have graphed the declining size of this projected range (in cm) to show how the science is moving towards more precise projections of SLC and diminishing uncertainty.
Different “types” of SLC
AR5 talks about a few different types of sea level rise. Relative Sea Level Rise is the height of the ocean at any given time with respect to the solid surface of the Earth. We measure this with geological records over long periods of times, as well as through tidal gauges. Geocentric Sea Level rise requires the use of satellite altimetry. Mean Sea Level Rise averages SLC in a certain period in time in a certain region. Most of the use of the word Sea Level Change in the IPCC AR5 is Mean Sea Level Rise. Global Mean Sea Level (GMSL) is the Mean Sea Level across the entire planet.
Processes that impact SLC
Processes in the oceans, atmosphere, hydrological cycle, and land-ice all interact to impact SLC. Overall, these processes cause sea level to vary on a broad range of spatial and temporal scales (from short lived events to changes over several decades).
I find sediment transfer from estuaries into our oceans to be one of the more interesting and less-talked-about processes (compared to oceanic currents, land-ice interactions etc.). Reason being, sediment deposit processes create more obvious SLC on a local scale (contributing about 0.01 mm to in SLC per year locally!) They are also interesting because it is quite difficult to make local claims on projected SLC, but with estuary based sedimentation effects it is possible.
Models Used to Interpret SLC
This IPCC chapter focuses on the models we use to compare predictions for SLC based on observation (from measurements) and on simulations (probability-based). Of primary importance are the Atmosphere Ocean General Circulation Models (AOGCMs or just GCMs). These models have components that represent the changes in the ocean, atmosphere, cryosphere, from both natural sources and anthropogenic sources.
There are many more models discussed in this chapter, but I find that the way they are classified is more worth discussing, versus getting into detail on each one. The first type, semi-empirical models, use probability and statistics to make predictions for SLC within models with set parameters from observations. These models do not simulate the processes that affect SLC. Process-based models on the other hand, do simulate processes.
Use of Paleo Data to diminish uncertainty in future projections
Another focus of the SLC chapter in the IPCC is using the Paleo Record to examine periods in geological history that had similar warming to what we are seeing today (1.5 to 3 degree C range) and the SLC that came with it. This section looks at several periods: the Middle Pliocene (~5-2 million years ago), the Marine Isotope Stage 11 (~400,000 years ago), the Late Holocene (~7,000 years ago), and the Instrumental Age (1700-today). Generally, each period of warming shows an accompanied rise in global mean sea level in line with what modern projections indicate we will meet in the coming centuries. Looking at past warming and accompanying SLC helps us to lower uncertainties with future projections.
One of the most reliable ways to examine paleo data is from salt marshes. Salt marshes are not only one of the most biologically productive ecosystems on the planet but they also give us detailed glimpses into the planet’s climate history.
Major Contributions to SLC in the Instrumental Age
In this section of IPCC chapter 13, the authors describe changes in GMSL from the 1700s to 2010. It is called the Instrumental Age, since we have been able to monitor SLC using tide gauges and satellite altimetry. The main focus of the chapter is reconciling what we observe empirically with what our best models predict.
One of the largest advances in getting better observed data comes from the Argo Project of a global network of floating censors dispersed throughout the world’s oceans to collect data on the changing climate and oceanic heat storage. In a recent lecture at Woods Hole Oceanographic Institute on the topic of the new IPCC report, Professor Raymond Schmitt, a senior physical oceanographer, argued that there is a large bequest value that we should place on sound data on the current state of the ocean. Also that we must be able to pass down such knowledge and high quality data to future generations seeking to manage the earth’s natural capital. The Argo Project is one of the best ways to ensure this happens, as we can see from the IPCC report.
In terms of numbers, the report stated that from 1993-2010 GMSL rise is consistent between observed and modeled predictions. The biggest contributors are oceanic thermal expansion and glacier mass lost. These two processes account for 75% of SLC. There is also evidence for faster rates of change beginning in the 1990s, and these are not due to natural variations, and we can make this claim with high certainty.