Paper: USGCRP, 2017: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 470 pp.
A new climate report released last Friday appears to have cleared federal review despite censorship fears. This has surprised some people, given that information about the science and consequences of climate change has been removed from a number of U.S. federal agency websites since the Trump administration took office. The report clearly that states humans are fundamentally changing the planet by pumping greenhouse gases such as carbon dioxide (CO2) into the atmosphere and ocean when fossil fuels such as coal, oil and gas are burned for transportation and energy.
The report is the 1st volume of the 4th National Climate Assessment, a government-mandated analysis of the causes and impacts of global warming that is due in 2018. Two more parts are in the works for review: an analysis on how climate change is affecting life in the U.S., and a summary of the latest findings on the global carbon cycle. Together, these three documents paint a newer, more dramatic picture of how global warming is affecting people and communities across the United States. Following the last edition in 2014, the heavily peer-reviewed assessment is coordinated by scientists from the National Oceanic and Atmospheric Administration (NOAA), the National Aeronautics and Space Administration (NASA), the Department of Energy, the Environmental Protection Agency, and the US Global Change Research Program, and draws from the most recent peer-reviewed research.
So what has changed since the last report in 2014? The new assessment highlights a list of areas in which our understanding has improved, including the evaluation of the human contribution to individual extreme weather events, higher-resolution climate models producing better simulations of things like hurricanes, and studies of ice loss in Greenland and Antarctica that have bumped the worst-case sea level rise estimates upward. It states with higher certainty that tidal flooding is accelerating in more than 25 coastal cities along the Atlantic Ocean and Gulf of Mexico. It addresses drought as well, saying “Large forest fires have become more frequent in the western part of the country, while warmer spring temperatures and shrinking mountain snowpack are combining to reduce the amount of water available to the region’s cities and farms.”
The report comes with 15 Chapters and Appendices that touch on different aspects of climate change. Online, each chapter comes with a header of several “key findings” that is accompanied by supporting evidence.
For us ocean nerds, we get two whole chapters dedicated to the subject, plus a chapter that specifically discusses Arctic ice.
Chapter 12: Sea Level Rise
This chapter discusses the physical factors contributing to Global Mean Sea Level (GMSL), Sea Level Rise (SLR), past sea levels, recent trends, and projected sea levels based on certain scenarios. The report’s SLR projections are frank regarding the uncertainty surrounding the worst-case scenario: while it projects 0.3 to 1.3 meters (1ft – 4.2ft) of SLR over the 21st century, it acknowledges that with high emissions scenarios and with new studies on Antarctic ice sheet stability, “eight feet by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.” This is a difference of nearly 4 feet from previous estimates.
- Human-caused climate change has made a substantial contribution to GMSL rise since the year 1900.
- Relative to the year 2000, GMSL is very likely to rise by 0.3–1.30m by 2100, possibly up to 2.4m under worst case scenarios.
- Due to changes in Earth’s gravitational field and rotation from melting of Antarctic land ice, changes in ocean circulation, and vertical land motion, sea level rise may actually be higher than the global average along all U.S. coastlines outside Alaska.
- Along with rising seas, the number of tidal floods each year that cause minor impacts such as blocking or damaging roads (also called “nuisance floods”) have increased 5- to 10-fold since the 1960s in several U.S. coastal cities. Rates of increase are accelerating in over 25 Atlantic and Gulf Coast cities. Tidal flooding will continue increasing in depth, frequency, and extent.
- It is also very likely that SLR will increase the frequency and extent of extreme flooding associated with coastal storms, such as hurricanes and nor’easters (a type of cyclone seen typically off the New England coast).
Chapter 13: Ocean Acidification and Other Ocean Changes
This chapter discusses human-driven changes to our oceans regarding chemical composition, temperature, and circulation of the global ocean. The researchers state that “Some of these changes will be distinguishable from the background natural variability in nearly half of the global open ocean within a decade, with important consequences for marine ecosystems and their services.”
- The ocean has absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-1900s, making waters warmer and altering global and regional climate feedbacks such as head flow between air and water. Ocean heat has also increased at all depths and surface waters have warmed by about 0.7°C per century worldwide. Under higher emissions scenarios, a global increase in average sea surface temperature of 2.7°C by 2100 is projected, with even higher changes in some U.S. coastal regions.
- Due to melting ice and increased freshwater in the oceans, the potential slowing of the Atlantic meridional overturning circulation (AMOC; of which the Gulf Stream is one component) could have dramatic climate feedbacks as the ocean absorbs less heat and CO2 from the atmosphere, affecting climates of North America and Europe. However, to date, this cannot be directly tied to human activity.
- The oceans are absorbing more than a quarter of the CO2 emitted into the atmosphere annually from human activities, where it reacts with seawater, making them more acidic, which harms marine life systems and communities that depend on them. Higher-latitude systems typically have a lower buffering capacity against pH change (measure of acidity), exhibiting seasonally corrosive conditions sooner than low-latitude systems. Acidification is regionally increasing along U.S. coastal systems as a result of upwelling (when deeper, colder water comes to the surface), changes in freshwater inputs, and nutrient input. The rate of acidification is unparalleled in at least the past 66 million years (since before the dinosaurs went extinct!).
- The oceans are losing oxygen, too: “Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing to overall declining oxygen concentrations at intermediate depths in various ocean locations and in many coastal areas.” Ocean oxygen levels are projected to decrease by as much as 3.5% under the higher emissions scenarios by 2100 relative to preindustrial values. Even though fish don’t breathe air, they need oxygen in the water to live.
Will this report change anything?
Like all scientific reports, this climate assessment does contain projection uncertainty. The majority of the uncertainty here is in the future trajectory of our emissions: “The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse (heat-trapping) gases emitted globally and on the remaining uncertainty in the sensitivity of Earth’s climate to those emissions.” Based on this, the report makes clear that climate change is a story about human actions—not natural cycles.
While the report doesn’t speak to potential climate policies for the U.S., it does explain that existing emissions pledges are not sufficient to limit global warming to 2° C, which has been a standing international goal. Unfortunately, few expect US government’s latest set of climate-change analyses to affect how the Trump administration approaches energy and environmental issues. Nor is it clear whether senior Trump administration officials will accept the reports’ core scientific conclusions. Nonetheless, many scientists and environmentalists lauded the new reports for bolstering the case for more-aggressive action against climate change.
Zoe has an M.S. in Oceanography and a B.S. in Geologic Oceanography from URI, with a minor in Writing and Rhetoric. She was recently a Knauss Marine Policy Fellow in the US House of Representatives, and now work at Consortium for Ocean Leadership. When not writing and editing, Zoe enjoys rowing, rock climbing, skiing, and reading.