Carbon dioxide and (steady state) climate change (3)

"When global warming was discovered?"

• Not a sudden event. Following Weart's narrative, a century-long event starting perhaps from 1896 (Arrhenius' paper).
• If (rhetorically) asked to specify an year, I will answer "1975".
  • Manabe & Wetherald's steady-state response with a 3-dimensional model (basically corroborating their 1-dimensional model in 1967) on "Journal of the Atmospheric Sciences
  • Schneider's review article on the steady-state response on JAS
  • Brocker's commentary "Climate change: Are we on the blink of a pronounced global warming?" on Nature

Development from consensus among experts to a politically relevant issue

• 1979: US NRC (NAS-related) "Charney report" ... steady-state response to CO₂ doubling : 1.5 ～ 4.5 K
• 1982: US NRC "Smagorinsky report" ... update of the above.
• 1983: US NRC "Nierenberg report" ... Discussion on impacts and countermeasures began.
• 1986: SCOPE report (Bolin et al.) (<- 1985 Villach [Austria] meetings of experts)
• 1988: Establishment of IPCC (Intergovernmental Panel on Climate Change) under UNEP and WMO. President: Bolin (U. Stockholm)
• 1990: IPCC First Assessment Reports
• 1992: UN Framework Convention on Climate Change

Today's subject: aspects within natural science. (Discussion about social and political factors ... postponed).

Explanation of global warming as slow transition between quasi-steady states

(Manabe, lecture in 1983, a Japanese publication 1985, redrawn by Masuda 2014)

Included in [Materials of 2019-May-23], but not explained yet.

Other factors than CO₂?

• Orbital parameters ... important in 10000-year time scale. not in 100-year scale.
• Solar activity ... maybe important, but unpredicatable.
• Volcanic aerosols ... significant cooling (by sulfates), but unpredictable.
• Anthropogenic aerosols (smog) ... locally important (cooling if sulphates), globally less important than CO2.
• Ozone ... itself important environmental problem. Connection to climate is complicated, but secondary.

How fast will our climate change?

Need to discuss time-dependent (transient) states rather than steady states

Two kinds of stock (reservoir)

• energy content of the climate system
• mass of CO₂ in the atmosphere

Energy balance of the climate system

• CO₂ concentration in the atmosphere -> contribution to net gain of energy by the climate system ("radiative forcing") ... immediate
• radiative forcing -> (surface air or troposphere) temperature ... need to consider energy storage

C dT/dt = F

• This is an approximation by (time-dependent) 0-dimensional energy balance model.
• C: heat capacity (per unit area of surface) [J/(m² K)]
• T: temperature [K]
• F: radiative forcing (per unit area of surface per unit time) [W/m²]

Problem: what comprises C?

Major reservoirs of energy

• 1. ocean heat capacity
  • (annual) mixed layer, depth ～ 100 m
  • upper ocean, ～ 500 m
  • deep ocean ～ 4000 m
• 2. ice sheets and glaciers (solid/liquid phase change)

Carbon cycle, esp. Mass balance of carbon dioxide in the atmosphere

• (geological exchanges ... slow)
• photosynthesis vs. decomposition of organic matter (incl. respiration) almost balanced in annual averages
• emission from fossil fuel combustion -> atmosphere and other reservoirs

Defelopment in the 1980s w.r.t. energy cycle and temprature

"transient response experiment" with 3-dimensional coupled ocean-atmosphere model by Spelman and Manabe (1984)

Idealized condition

• Idealized geography, half-ocean, half-land
• No seasonal cycle
• Sudden 4-fold increase of CO₂ concentration

See [Delay due to oceanic heat capacity]

Main result: Upper ocean warms together with the atmosphere. The time scale is decades (tens of years).

Implications:

• Global warming due to increased carbon dioxide will be significant in several decades.
• If we can stop emission, the effect will continue for several decades.
• Such stories can be refuted:
  • "Global warming will be very slow because the deep ocean has large heat capacity."
  • "CO₂ is unlikely to be the cause of climate change because the time series of temperature does not look like that of CO2 concentration."

More realistic experiments followed.

• time-dependent response to 1%/year increase (compound interest) of CO₂ concentration
• (multiple) realistic scenarios of future CO₂ concentration
  • e.g. Hansen et al. (1988) -> Hansen's testimony at US Congress in summer 1988

Carbon cycle

Empirical fact (as of 1970s-1980s)

• Increase of mass of CO₂ in the atmosphere ～ half of emission by fossil fuel combustion.
• The remaining half must have gone to {ocean+land}.

Difficulty in explaining "sinks" (=negative sources)

• terrestrial ecologists ... obvious source of CO₂ from deforestation in the tropics
• marine chemists ... issue of solubility of CO₂ in seawater and shifting carbonate chemical equilibrium (HCO₃^-, CO₃^2-) etc.)

Tentative assumption (common as of 1980s)

• The fraction of {increase in the atmosphere}/{emission} remains constant in the future.

later finding (1980s-2000s)

• land ... "sink" of CO₂ in boreal (cold temperate) forests, helped by CO₂ fertilizetion (not climate in a narrow sense)
• ocean ... "sink" of CO₂, sinking particulate matter (CaCO₃ + organic) has some role

Both are not gurranteed to continue at similar fraction in the future.

• land ... warmer climate -> higher rate of decomposition of organic matter, which may exceed gain in photosynthesis
• ocean ... higher CO2 concentration -> higher solubility of CaCO₃ -> reduce activity of organisms that create CaCO₃ shells

Impact on climate change on agriculture (and food), forestry, and natural ecosystems

Scientific studies were promoted in 1980s.

"Something bad may happen. International policy may be needed." -> momentum to establish IPCC etc.