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Climate change 2021 the physical science basis

The IPCC 6th Assessment Report: the Truth, Plain and Not So Simple

The IPCC report was released on August 9th. If you don’t have the patience to read the 1300 page document or even the 40-page executive summary, here are select take-aways.  More may follow in a future newsletter.

The Intergovernmental Panel on Climate Change’s (IPCC) 6th Assessment Report issued earlier this month begins with the strongest language to date, stating that “It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.”  This statement is subsequently supported with quantified measures of greenhouse gas concentrations, global surface temperature data, over-land precipitation data, glacial ice retreat, and global sea level rise.  On each measure, the facts speak for themselves, together they validate what scientists, and now most citizens, have accepted for years. Human activity, primarily fossil fuel, methane and nitroxide emissions, are warming the planet at an unprecedented rate.

Key metrics to note, and be able to quote confidently include:

  • In 2019 the atmospheric CO2 concentration reached 410 ppm, higher than any time in at least 2 million years, and the concentration of CH(methane) and N2O (nitroxide) were higher than any time in the past 800,000 years.
  • Global surface temperatures increased faster from 1970 through 2020 than in any other 50-year period in the last 2000 years. The temperatures during the last decade from 2011 through 2020 were warmer than any multi-decadal warming period in 6500 years.  The last time it was this warm was 125,000 years ago.
  • The rate of mean sea level rise since 1900 is faster than any century in 3000 years. 50% of the increase from 1971 through 2018 came from thermal expansion, while 22% came from melting glaciers, 20% from melting ice sheets, and 8% from a decrease in land storage. However, the amount coming from ice sheets and glacier mass loss increased by a factor of four in recent decades.

Like the previous report, AR5, this report, AR6, reaffirms with high confidence that there is a near-linear relationship between cumulative anthropogenic CO2 emissions and the global warming they cause. The assessment report also makes clear that projected climate impacts, including frequency and intensity of hot extremes, marine heatwaves, heavy precipitation, agricultural and ecological droughts, the proportion of intense tropical cyclones, as well as reductions in arctic sea ice, snow cover, and permafrost all become larger in direct relation to increasing global warming.  Even a 0.50 C increase in temperatures sets these multiple impacts in motion more intensely.  In particular, scientists are ‘virtually certain’ that land surfaces will continue to warm more than the ocean surface and that arctic warming will be greater than that of the overall global surface.

The report describes the impacts of five different emission scenarios ranging from a rapid decrease in emissions to net-zero by 2050 and negative emissions following (SSP1-1.9 and SSP1—2.6), to a pessimistic model that assumes roughly a doubling of emissions between 2050 and 2100. The intermediate scenario, (SSP2-4.5) which is probable given current growth paradigms and entrenched structural economic relationships and regulatory frameworks, models current levels of emissions through 2050, with waning emissions thereafter.  The likelihood and level of intensity of different types of impacts are carefully evaluated for each model, and the conclusions posited in the report are qualified with statements indicating the level of certainty or confidence with which the scientific team makes statements about the future conditions.  In the SSP2-4.5 model, temperatures in the latter part of the century would rise by 2.1oC to 3.5oC, a level that would radically and negatively impact human civilization. We cannot proceed on this trajectory, and therefore must quickly reorganize social and economic activity on a sustainable path.

It is interesting that the IPCC’s executive summary matter-of-factly addresses observed anomalies, variations in data,  and uncertainty in projections, some used by climate deniers to cast doubt on the scientific validity of the conclusions. They state that a warmer climate will intensify very wet and very dry weather and climate events and seasons, with implications for flooding or drought (with high confidence), but the location and frequency of these events depend on projected changes in regional atmospheric circulation.  Natural internal variability of precipitation patterns that are observed in decadal intervals in specific regions often mask, and will continue to mask, directional trends in the near future.  It is imperative to understand that these episodic events do not mean that the overriding trend is not happening.  For example, a large volcanic eruption may provide a temporary cooling impact in some areas for a period of one to three years, but the impact of the volcanic activity does not mean that more heat is still being captured by the global earth system.  Once the temporary effect dims, temperatures and precipitation will return to the prevailing trend line. Likewise, the rate of warming is masked a bit by a cooling effect from a breakdown of ozone resulting from aerosol release. This cooling factor will not last indefinitely either, and as it fades, heat will increase from the underlying concentrations of greenhouse gases.

The report also provides information that should shape how we solve the problem.  The modeling suggests that under the higher emission scenarios, solutions related to carbon sinks in land and oceans will not be sufficient to effectively manage emissions.  We simply must cut the emissions as expediently as possible.  Even if we cut emissions now to meet the trends projected in the low emissions model (SSP1-1.9), changes in ocean temperatures, ice sheets and global sea level rise will persist for centuries, perhaps a millennium, so we must plan to adapt to changing conditions in a way that allows humans to create resilient settlements and societies.  Moreover, we must plan for compounding impacts.  The likelihood of experiencing multiple climate impact events concurrently is growing and gets worse in higher temperature scenarios. So if rising sea levels are driving populations to high ground and inland locations, one must also consider that those inland areas may experience drought conditions, limiting water supplies for the growing population. If global warming increases, some compound extreme events, (events of increased intensity, duration of geographic extent) with low likelihood in past and present will become more frequent.  This suggests that adaptation planning and associated engineering specifications may need to adjust to conditions not anticipated previously.

Planners must consider how compounded climate-impact drivers affect cities as 56.2% percent of the world’s population lives in urban areas, a number expected to rise to 68% by 2050, according to UN projections.  Urbanized areas tend to have more extreme heatwaves because asphalt, buildings, and the lack of greenery produce a heat-island effect. The built environment of urbanized areas can also cause increased precipitation over the city and downwind, suggesting that these areas should be planned to absorb and store water. Open public spaces with permeable surfaces, greenery, subterranean cisterns; vegetated buildings, elevated transportation, shaded pedestrian areas are some adaptive techniques.   Whereas planning and zoning regulations have historically managed density in relation to infrastructure capacity, commercial value, and aesthetic preferences, going forward a new paradigm that prioritizes urban climate resilience must prevail. Resilience stemming from a harmonious relationship with ecosystems and incorporating concerns for social equity must supersede the approach rooted primarily in the capacity of engineering to fend-off risks.

For at least the last 25 to 30 years, climate and environmental scientists, and a growing share of the public, have been aware of the threats a changing climate poses to human civilization. Yet, there has been a prolonged apathy to act aggressively on both mitigating emissions and adapting to the emerging impacts.  The needed changes appear to require too much sacrifice, both personal and collective. Moreover, there seemed to be plenty of time.  Climate change would happen in the future. So, so long as we gradually improved recycling, bought more organic food, and remembered to turn off the lights when we left the room, everything would be okay. The IPCC’s 6th Assessment Report tells us in plain terms, we were wrong. Time is up! Greenhouse gas emissions, both carbon dioxide, methane, and nitroxide must drop to net zero in less than two decades! It now falls on us to collectively support climate action and make use of existing technologies and information to take immediate steps in the right direction.






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