The Physical Science of Climate Change: A Summary   Leave a comment

The physical science of climate change has made remarkable progress since the last IPCC Working Group 1 report. Recall that Working Group 1 publishes a report on the physical basis of climate change, Group 2 on the vulnerability of people and natural systems, and Group 3 on strategies for solving the climate crisis. Only report 1 has been released; the other two are pending. The previous IPCC reports (AR5 vs. AR6) were published in 2013.

I’ll summarize the key points of the group 1 report, which is based entirely on peer-reviewed science up to January 31, 2021. I’ll also use updated information and emphases from the IPCC presentations at the climate conference. As you might expect, there’s been even more progress since late January, including a few compelling studies about ice in Antarctica. But let’s just focus on report 1 here. My summary is selective. I may well leave out elements that you’re particularly interested in. But there’s too much for me to summarize everything here.

For many of you, the best place to view these results is the Report for Policymakers. You can challenge yourself a little more with the Technical Summary. And, if you want all the details, seek out the full report, which is comprehensive and long—well it should be given the subject matter. If you read any of the reports, you’ll notice that the scientists have provided an assessment of the level of confidence for each conclusion, which allows readers to weigh the certainty. I’m going to place a few of the graphics from the Technical Summary at the bottom. If you’d like to see more, check out one of the reports at https://www.ipcc.ch/report/ar6/wg1/.

So, let’s dive in.

(1) What’s happened so far?

THE BASICS. The average annual global temperature in 2010-2019 was 1.1 deg C ( 1.8 deg F) warmer than in 1850-1900. That might not seem like a lot, but it’s enough to melt glaciers and ice sheets, raise sea level, lead to dangerous heat waves and droughts, and increase the water holding capacity of air, which is the energy force for major storms. Each of the last four decades was successively hotter than any previous decade since 1850 — that’s rather astounding. It has just gotten hotter and hotter and hotter for the past more than ½ century. In addition to temperature rise, global average precipitation has likely increased over the past several decades.

CAUSES OF WARMING. It is scientifically unequivocal that human activities have caused an increase in the temperature of the land, air, and oceans. Emissions from the burning of fossil fuels and other activities are responsible for these changes. These emissions continue to raise the concentration of greenhouse gas in the atmosphere, which has a heat trapping effect. Those atmospheric concentration of CO2 has continued to rise since the 2013 report. In 2013, it was 395 and in 2021it is 410 part per million (ppm). Increased global average precipitation also is likely the result of fossil fuel burning.

OCEANS. It is extremely likely that ocean water temperature has increased and pH become more acidic because of human fossil fuel burning. This has depleted oxygen in some areas, as deep as 700 m (2300’). Increased acidity speeds up the breakdown of marine shelled organisms and has other far-reaching effects on these ecosystems. Sea level has risen on average 0.2 m (about 8’) since 1901 and this rise has been accelerating (high levels of confidence).

ECOSYSTEMS. There is high confidence that climate and ecosystem zones have shifted poleward in response to warming.

(2) Comparing these changes to the distant past

These changes in the climate are unprecedented in terms of the following:

a. The level of atmospheric CO2 is higher than any time in over the past 2 million years (high confidence)

b. Temperatures in 2011-2019 were the highest since 125,000 yrs ago (medium confidence)

c. The retreat of glaciers and sea ice was unprecedented in the past more than 1000-2000 years (medium confidence)

d. In almost region, the frequency of heat waves has increased, cold spells decreased, and the intensity of precipitation increased.

e. There has been an increase in combined heat waves and droughts, fire weather, and flooding (medium to high confidence)

Again, these changes are attributed to increased atmospheric greenhouse gases. The extra heat trapping effect of these additional greenhouse gases has been estimated as 0.43 watt for each meter square of land (an area about 3 1/4 ‘ x 3 1/4’ = 10.5 sq ft) compared to the past.

(3) What are possible scenarios for the future?

There are two major steps to projecting the climate of the future. The first is to develop a range of future socioeconomic scenarios. These range from scenarios in which human civilization drastically cuts greenhouse gas emissions (net zero for example) to ones where greenhouse gas emissions actually rise in the future. Each scenario is given a number.

Scenario 8.5 is terrible, assuming more than a tripling of greenhouse gases emissions per year by 2100. Scenario 4.5 is much more encouraging, assuming a drastic cut in yearly emissions, although not quite down to zero. Scenario 1.9 assumes a rapid drop to negative emissions (i.e., net uptake from the atmosphere) by 2050 and then continual drops by 2100. These different scenarios — there are a total of five of them — are then used to run the complex climate models that provide projections of future atmospheric concentrations of greenhouse gases and changes in the climate (temperature, sea level, storms, etc.). I’ll stick to just those the scenarios 1.9, 4.5, and 8.5.

TEMPERATURE. For temperature, scenario 1.9 projects less than a 1.5C rise by 2100. Yeah! Scenario 4.5 almost 2.8C. Unacceptable.  Scenario 8.5 more than a 4 deg C rise. Truly frightening! The bottom line here is that the Earth will bypass a rise of both 1.5 and 2 degrees during the 21st century unless drastic reductions are made in greenhouse gas emissions.

THIS IS PERHAPS THE KEY FINDING OF THE GROUP 1 REPORT!

OTHER CLIMATE FACTORS. The other climate shocks experienced already this century would be exacerbated by the higher emissions scenarios, 4.5 and 8.5, and would eventually abate under 1.9. A key point of the report is that each unit of greenhouse gas released into the atmosphere will translate into not just higher temperature but also more extreme climate  in terms of heat waves, droughts, more intense storms, etc.

SEA LEVEL RISE. Not surprisingly, sea level rise is projected to be highly sensitive to differences in future emissions, although the effects accumulate more slowly than for temperature, storms, etc. This is because it takes a while for additional heat to melt ice, which then causes sea level rise. In all cases, sea level is expected to continue to rise until 2100. In the 1.9 scenario by 0.5 meter (20”), in the 4.5 by about 0.8-0.9 (32-35”), and for 8.5 an astounding 1.7 m (67-68”) by 2100. Imagine an additional 5 1/2 feet of sea level in your favorite low-lying coastal town. Average sea level rise is only part of the story, as well, given that a sea level rise of 3 feet for example means storm surges much higher than that.

It’s important to also describe a low-likelihood possibility, which is that a scenario such as 8.5 could actually change the dynamics of ice (shelves and sea ice) leading to extreme sea level rise by 2300, as much as 7 m or nearly 23 feet. That would wash away many large coastal cities. A few country representatives wondered if it was useful to present such low-likelihood, absolutely terrifying scenarios such as this, given that our goal is promote hope. The IPCC responded that it’s important to keep in mind all scenarios. This is especially true for those involving ice, which is perhaps the most uncertain component of the Earth system when it comes to climate change. This is, in part, because sea level rise from melting ice involves not just the thermodynamics of heat melting ice, but also the physical dynamics of ice sheet and sea ice as they shift and break up. To backtrack a bit, a large portion of the emissions are now being taken back up by the ocean and land, with a lower share staying in the atmosphere. This would also be the case with low emission future scenarios (e.g., 1.9 –> only 30% stays in the atmosphere). But the land and ocean reservoirs have only so much capacity, and with higher emissions scenarios, a larger portion will stay in the atmosphere (e.g., 8.5 –> 72%), warming the planet. This is a scary amplifying effect of high emissions scenarios.

Here are the five key messages of report 1 in my view. Others might see other key conclusions not included here.

(1) THE EARTH HAS UNEQUIVOCALLY WARMED, WHICH IS CREATING CLIMATE SHOCKS

(2) THE CAUSE IS UNEQUIVOCALLY HUMAN FOSSIL FUEL BURNING AND OTHER ACTIVITIES

(3) THIS IS CHANGING MANY ASPECTS OF THE CLIMATE AND EARTH SYSTEM

(4) EACH UNIT OF EMITTED GREENHOUSE GAS HAS A CORRESPONDING EFFECT ON THE CLIMATE SYSTEM, AND THIS EFFECT IS HIGHER FOR HIGHER EMISSIONS SCENARIOS (see last full paragraph above).

(5) TEMPERATURE WILL RISE DANGEROUSLY BEYOND BOTH 1.5 AND 2 DEG IN THE 21ST CENTURY UNLESS DRASTIC REDUCTIONS ARE MADE IN NET EMISSIONS

(6) 1-5 ARE CLEAR THAT WE MUST AVOID FUTURE HIGH EMISSIONS SCENARIOS.

Graphs below illustrate most of these conclusions.

These are the five future socioeconomic scenarios, in terms of carbon dioxide emissions
Here are the projected temperature increases based on each of the five future socioeconomic scenarios – table form.

Here are the projected future temperatures based on the five scenarios – graphical view.
Projected future sea ice based on the five scenarios
Projected future ocean pH based on five scenarios.
Sea level projections based on five scenarios, including low-likelihood sea level rise with scenario 8.5
Higher emissions scenarios involve not just higher emissions but also a higher portion of those emissions staying in the atmosphere and warming the planet.

Posted November 10, 2021 by changingnatureofthemainewoods in Uncategorized

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: