Winter 2017 — #23

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What will it take to keep Earth from warming 2 degrees C?

Hard work to throttle down carbon dioxide exhausts now begins

     Nearly all (197) nations agreed in 2015 to take actions to prevent global warming from surpassing 2 degrees Celsius (C) above temperatures that prevailed before the industrial revolution. But to meet that goal, nations will need to cut their emissions of greenhouse gases in 2030 more than they have committed to do. Worse, society will need to actively remove these gases, especially CO2, from the atmosphere: by 2085 more exhaust gas must be removed than will be emitted. The sooner human society begins to ramp down their exhausts of CO2, the less gas needs be removed and stored underground later.

     In the historic Paris agreement1 of 2015, nations agreed to limit the overall warming of the Earth by 2ºC or less (about 4º Fahrenheit). Last November, it went into effect when 125 nations committed to reduce their emissions of greenhouse gases (GHG) by the year 2030.

     The 2º target refers to all warming, past and future, caused by human influences since “pre-industrial” times, when developed nations began to burn fossil fuels (coal, oil, peat, and gas). Exhaust gasses from fuel burning and other industrial and commercial activities have already warmed the Earth by 1ºC (1.4ºF). That leaves only one more degree of “wiggle room” before further warming is supposed to end.

   The pathway society will follow toward a stable global temperature is not at all clear. Let's review a proposal from scientists to chart a path to a future climate that will remain less than 2º warmer than pre-industrial times.

     Sanderson and other authors2 (at the National Center for Atmospheric Research, NCAR) asked, What would it take to achieve the 2º and the 1.5º limits for acceptable warming?                       (continues below)

    Eight pathways that society could take to have a two-in-three chance of limiting warming to 2º Celsius or less.  The blue line represents our current rate of emissions. On the red line, society will be reducing emissions if countries keep their commitments to the Paris Agreement. Other possibilities (gray lines) require faster cuts in emissions in the near term, but fewer “negative emissions” later.     ©UCAR; available for media & nonprofit use.

     Their answer: Rapid cuts, starting in the next ten years, in greenhouse gas exhaust are needed to avoid much greater, very costly reductions later in this century, and to avoid a need to withdraw massive amounts of CO2 from the atmosphere and store it underground. Such withdrawals of CO2 are sometimes called “negative emissions.” Since the technology hardly exists now on a commercial scale, it may not become feasible to remove massive amounts of CO2 after 2050. “Small changes now equal big benefits later,” said Laura Snider, a senior science writer of NCAR, when she summarized Sanderson's findings3. As CO2 released today continues to accumulate and warm the Earth for well over a hundred years, it pays to emit less of it in the next 10 years and avoid drastic removal measures (like negative emissions) after 2050.

     The tradeoff can be seen in the figure (above, right) from Sanderson's paper, as redrawn by Snider. The blue line depicts the steeply climbing rate that greenhouse gases have been emitted in recent years. This rate shows no sign of slowing. After 2015, several curves depict possible paths for meeting the warming goal of 2º or less; each path has a 2-in-3 chance of succeeding. The red curve traces how emissions will fall off if all nations keep their commitments to emit less greenhouse gas in 2030. In that year, however, total emissions would drop to only 3% less than today's level. Eventually, late in this century, “negative emissions” will be needed to keep the warming below two degrees, when more CO2 has to be removed from the air than will be added to it.

     Sanderson adds that if nations can manage to reduce current emissions by 10% instead of 3% by 2030, there will be much less need for carbon capture later. The negative emissions required wouldl be about the same amount projected to occur in the lowest scenario (pathway to the future) of the Intergovernmental Panel on Climate Change.

     The pathways in the figure diverge after 2015, but (hello!) we are already past that year. Realistically, if we begin emission cuts in 2020, then “net emissions” must fall to zero by 2060, to keep global warming below 2º, or by 2043 to keep warming below 1.5º.

     Sanderson's team found easier paths to the desired climate goals if we cut ourselves some slack. Nations could agree to allow the global temperature to surpass the 2º (or 1.5º) goal temporarily for 50 years, after which time the planet cools back down. If the world overshoots the 2º target for 50 years, the total warming remains below 2.2º at all times, and then it falls. If we adopt the 1.5º target but overshoot it, warming still remains less than 1.8º, and we reach “net zero” emissions about 10 to 20 years later than otherwise.

     It will not be possible to limit warming to less than 1.5º unless nations rapidly reduce emissions of greenhouse gas in the next ten years, the authors conclude. The chances of attaining either temperature target are very sensitive to when society starts to cut these emissions. They state, “A 1.5º world and even a 2º world . . . can be attained with substantial and prompt global action, and each year . . . . that we follow the current high-emission path . . .reduces the chances that we achieve either temperature target.”


1. Paris Agreements (2015), Durban Platform for Enhanced Action (Decision 1/CP.17), Adoption of the Paris Agreement FCCC/CP/2015/L.9/Rev.1 . . . United Nations Framework Convention on Climate Change, Secretariat, Bonn, Germany.

2."What would it take to achieve the Paris temperature targets?” by Benjamin Sanderson, B.C. O'Neill, and C. Tebaldi (2016), Geophysical Research Letters, vol. 43, p. 7133.

3. News release, National Center for Atmospheric Research (NCAR), by Laura Snider, Jun. 27, 2016.

Toxic shellfish tied to warmer water on US west coast

      Since 1987, shellfish caught for food along the Pacific coast of North America sometimes contained a toxic acid that can poison people and wildlife that eat the seafood. Graduate student Morgaine McKibben and researchers at Oregon State University now show1 that the toxic seafood appears in coastal waters that are warmer than normal, during the warm phases of two natural climate cycles that affect the Pacific Northwest and California. As the oceans continue to warm up, such warm episodes are expected to occur more often.

     One variety of single-cell diatom (Pseudo-nitzschia) is common in sunlit coastal surface waters. Diatoms are important microscopic algae that generate, through photosynthesis, about 45% of all organic matter in the oceans2 ; thus they play a large role in oceanic ecosystems. But this variety of diatom also produces the neurotoxin domoic acid. Certain shellfish and fish such as anchovies accumulate the toxic acid in their flesh after they eat the diatoms. When humans eat seafood containing domoic acid, they can experience Amnesic Shellfish Poisoning, which causes intestinal disorders, memory loss, and rarely death. Since 1991, Oregon has monitored shellfish for the presence of domoic acid, and, when present, halted the sale of seafood for public health reasons. Large marine mammals including sea lions, sea otters, dolphins and whales have died during toxic episodes.

      A long period of monitoring over the past 25 years has allowed scientists to link the outbreaks of toxins in seafood to two natural climatic cycles in the North Pacific Ocean: the Pacific Decadal Oscillation (PDO) and the El Niño / Southern Oscillation. Both climate variations set up similar patterns of warm and cool water regions in the northeast Pacific.

      While three west coast states were monitoring the toxin, five warm phases of either the PDO or El Niño oscillations occurred. In every one of these five, domoic acid was found in the flesh of razor clams in Oregon (or in other seafood in other states) in quantities greater than 20 parts per million. When discovered, that quantity triggers a seafood ban in Oregon. In every event, McKibben found the toxin in seafood from at least two of the three states.

      Climatologists use indices to decide when a climate cycle begins and ends, and whether it is in a warm or a cool phase. Domoic acid in seafood correlated better with a combined index for the PDO and El Niño/La Niña, than with either index alone.

     Pacific Northwest coastal waters go through seasonal shifts from downwelling (sinking) water to upwelling (rising) water sometime in the spring or summer, or even as late as the autumn. Even though the presence of toxins was closely linked to warmer-than-normal surface water, it also was linked to cold upwelling water because plankton is fertilized by nutrients arising from the deep ocean. Toxic amounts of domoic acid in shellfish occurred after a warmer-than-normal downwelling season, and during the upwelling season when plankton was nourished by high levels of nutrients.

      Movement of warm water masses from the open ocean onto the continental shelf also favored toxic outbreaks. The warmer water was then situated over the shelf where subsequent upwelling delivered the nutrients for a “feeding frenzy.”

What environmental conditions seem to bring toxic algae? The PDO and El Niño indices, which are highest in the warm phase of the cycles, correlated well with warm water in the harbor of Newport, Oregon, from December to March––before the spring or summer transition to upwelling. When the warm water drifted onto the coast from the open ocean or from the south, it brought warm-water varieties of algae, including some with toxic domoic acid. The conditions were in place to foster the growth of toxic algae once nutrient-rich water arose from the deep. Another outbreak of toxic shellfish becomes likely.


1. "Climatic regulation of the neurotoxin domoic acid" by Morgaine McKibben et al., 2017: Proceedings of National Academy of Sciences, vol. 114, no. 2, 239-244, doi: 10.1073/pnas.1606798114.

2. Wikipedia.

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