What is the Fourth National Climate Assessment?

, senior climate scientist | November 23, 2018, 1:57 pm EST
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The congressionally-mandated Fourth National Climate Assessment (NCA4) is the culmination of an enormous effort by the U.S. government and scientists from a variety of sectors to provide the U.S. public with the state of the science on climate change, the impacts that it is currently having on the country, and those that are likely to manifest.

The report also provides information on the measures that the country can take both to cope with current and coming impacts and to lower global warming emissions – this information can aid in risk assessments and help prevent the most dangerous impacts of climate change. As the report is a scientific assessment, it does not provide policy recommendations. Instead, it provides decision-makers with objective information that they can use to decide how to best protect their constituents.

What’s the difference between Volumes I and II?

The U.S. Global Change Research Program (USGCRP) released the Climate Science Special Report (CSSR), billed as Volume I of the Fourth National Climate Assessment, in November of 2017. The CSSR provided information on how and why the U.S.’s climate is changing – from the human fingerprint on observed increases in extreme heat, to more frequent large wildfires out West, to more intense hurricanes in the North Atlantic since the 1970s. The report also provides the U.S. public with information on how the climate is likely to change further in coming decades.

Volume II of the Fourth National Climate Assessment that was recently released builds upon the CSSR and focuses on the impacts and risks associated with climate change for U.S. sectors (including agriculture, health, energy, and transportation) and regions (such as the Southeast, the Northern Great Plains, the Northwest, and the U.S. Caribbean).

Volume II further assesses risks and impacts for indigenous peoples, as well as the implications of international climate impacts for U.S. national security interests. It also examines options for the country to adapt to climate change, as well as lessen risks and impacts by reducing global warming gases (mitigation).

What’s new?

Many of the top-line findings are not new–namely that climate change is here and now, is caused by humans, and is expected to pose increasingly serious consequences for each of us, especially if we do not take action now.

The report, however, covers new ground by, for example, providing greater specificity on the economic impacts that climate change is likely to have. As an example, the NCA4 assesses that each year, some U.S. sectors are likely to see more than $100 billion of losses by the end of this century as a result of climate change, which, for context, is higher than the GDP of many states. (For a more detailed review of economic impacts, please see my colleague Rachel Cleetus’s blog).

Relative to NCA3, there is now information available on the interconnectedness of different sectors, and how this can lead to ripple effects. One can think of the extreme heat waves that hit many parts of the country this year, causing power outages that affected both homes and businesses alike.

The NCA4 further highlights the consequences of climate change that we are facing. For example, many regions are likely to experience reduced crop yields, as well as reductions in the quality of the crops that they are producing and their grazing lands in a warming world.

Sea level rise and associated flooding poses threats to an estimated $1 trillion in coastal real estate. And sadly, the most vulnerable among us – such as our youth and our elderly – are most likely to be affected by climate change given their disproportionate vulnerability to, for example, extreme heat events that are becoming increasingly common.

NCA4 in Context

NCA4 is coming out on the heels of the Intergovernmental Panel on Climate Change (IPCC) Special Report on Global Warming of 1.5°C. The IPCC’s report provided the world with the closest look to date at how climate change is likely to manifest in the near-term, and the measures necessary to limit warming to 1.5°C and 2°C–the targets of the Paris Agreement. That report made it clear that while a certain level of warming is locked in, a wide range of serious impacts can be avoided–from hundreds of millions of additional people being regularly exposed to extreme heat waves worldwide to the complete loss of warm water coral reefs – if we can contain warming to 1.5°C instead of 2°C.

Achieving either of these temperature targets will require transformations in many aspects of our society, including the way we use and produce energy, the way we grow and consume food, and how we transport ourselves.

The NCA4 underscores these points for the United States and shows that we can pave the way towards a safer future if we take immediate action to reduce global warming emissions. There is hope in the commitments that communities, states, and businesses around the country are making, and at the same time, a lack of federal leadership makes achieving this brighter future more difficult. There is so much room for common ground in this arena–from shared experiences of extreme events to shared opportunities from clean energy investments–it is incumbent on our leaders to come together and find a path forward that reflects this best available science.

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