Surface–Climate Feedback Processes SOEE3410 : Lecture 6 Ian Brooks ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics

SOEE3410 : Atmosphere and Ocean Climate Change Processes The CO2 Budget N.B. Gross annual exchange between ocean and atmosphere is: A large fraction of total atmospheric storage (16%) Much larger than human output A small imbalance in exchange terms could result in CO2 as large as anthropogenic emission. It is thus important to understand how natural exchanges may respond to changes in climate. CO2 storage terms (PgC) Ocean : 38,000 Atmosphere : 730 Soil : 1500 Plants : 500 Anthropogenic output (PgC/year) 1980’s : 5.4 0.3 1990’s : 6.3 0.4 Gross exchange of CO2 (PgC/year) Land-atmosphere : 120 Ocean-atmosphere : 90 Units: 1 PgC = 1 Petagram of Carbon 1 Pg = 1 × 1015 grams = 1 × 1012 kilograms = 1 × 109 tonnes SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Measured rate of increase in atmospheric CO2 (emission–uptake): 1980-1989 : 3.3 0.1 PgC/year 1990-1999 : 3.2 0.1 PgC/year These rates are variable on a year-to-year basis 1992 : 1.9 PgC/year 1998 : 6.0 PgC/year Variability is due to variations in uptake resulting from large-scale annual variability in climate processes: e.g. the El-Niño Southern Oscillation and North-Atlantic Oscillation SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes The land and ocean uptakes in CO2 can be distinguished from measurements of CO2, O2 and isotopes18O, 13C. The uptake is the difference between gross quantities absorbed and emitted. The estimated uptakes are (PgC/year): 1980-1989 1990-1999 Land-atmosphere flux: -0.2 0.7 -1.4 0.7 Ocean-atmosphere flux: -1.9 0.6 -1.7 0.5 SOEE3410 : Atmosphere and Ocean Climate Change Processes

CO2 Cycle Feedbacks (Land) Higher atmospheric CO2 concentration: Fertilisation effect increases rate of Net Primary Production Leaf stomata partially close, reducing water losses; increased water use efficiency also increases NPP Effectiveness of biomass as a sink of CO2 depends on: Conversion of CO2 to carbon compounds with long residence times: wood, modified soil organic matter. NPP may not continue to increase with ever higher CO2 concentrations. CO2 must return to atmosphere eventually – possibly a new equilibrium higher total biomass. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Warming of environment: On short time-scales: increase in heterotrophic respiration (especially decay processes) increases, increasing return of CO2 to atmosphere. It is not known what the long-term effects on net land-atmosphere exchange are. Changes to regional cloud cover and precipitation are likely to affect regional ecology. This may act locally to promote or reduce CO2 uptake. SOEE3410 : Atmosphere and Ocean Climate Change Processes

CO2 Cycle Feedbacks (Ocean) Higher atmospheric CO2 concentration: Flux of CO2 between ocean and atmosphere is driven by pCO2  while atmospheric concentration increases it will drive increased ocean uptake…but… Fraction of anthropogenic CO2 that is taken up by ocean decreases as the concentration increases due to reduced buffering capacity of the carbonate system Fractional uptake also decreases because it is ultimately limited by the slow exchange of surface and deep-ocean waters Surface waters reach equilibrium with atmosphere in ~1 year – much faster than exchange of surface and deep water. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes There is NO fertilisation effect in the ocean With limited exceptions photosynthesis takes place via different reaction pathways than for land-based plants, CO2 availability is not a limiting factor. Higher CO2 concentrations decrease pH of ocean waters – may change ecology and rate of calcification. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Warming of ocean surface waters: Reduces solubility of CO2 in water, reducing uptake Increases vertical stratification of the water column: Reduces outgassing from upwelling regions (reduced upwelling) Reduces mixing of surface waters with deep-ocean water, thus reducing rate of uptake Changes to bioproductivity / regional ecology Changes to rate of calcification Increased phytoplankton production of DMS has links to aerosol & cloud properties (see marine cloud – aerosol feedbacks) SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes ALL models suggest that the net effect of climate feedbacks on the carbon cycle is to reduce the rate of uptake and increase atmospheric CO2 concentrations. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Water Vapour Feedback Within the boundary layer relative humidity tends to remain ~constant, and water vapour content increases with temperature. Note non-linear increase of saturation vapour pressure with T. Within the free troposphere water vapour content cannot be inferred from simple thermodynamic arguments. It is controlled by complex dynamic and micro-physical processes that are not all well represented by current models. Water vapour is a strong greenhouse gas. It’s effect is most important in the Free Troposphere, above the boundary layer. An increase in water vapour here would lead to further warming – a strong positive feedback. This is the most important reason for large responses to increased greenhouse gases. SOEE3410 : Atmosphere and Ocean Climate Change Processes

Marine Cloud – Aerosol Feedbacks Marine stratocumulus cover very extensive regions – large, persistent cloud decks off the western continental margins. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Marine stratocumulus are an important influence on the radiation budget, reflecting solar radiation and limiting longwave losses at night. Cloud reflectivity is sensitive to the droplet size and number distribution – this is determined by the available water vapour and cloud condensation nuclei (CCN) concentrations. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Shiptracks: This enhanced satellite image shows tracks in low-level marine stratocumulus clouds. Aerosols in ship exhaust act as CCN, increasing the number of droplets in the cloud (since the water available is the same, this reduces the mean droplet size). The change in the droplet spectrum makes the cloud more reflective. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes It has been calculated that a 2% increase in cloud albedo would offset the warming resulting from a doubling of CO2 (Twomey et al. 1984). Marine clouds exist in a relatively clean environment (low aerosol concentrations). Large changes in reflectivity can be produced by modest changes in CCN concentration. Anthropogenic aerosols: Significant over land, and where flow is from land to sea. Extensive marine stratocumulus regions are primarily in clean oceanic air masses Changes in natural aerosol production in response to changes in climate Twomey, S. A., M. Piepgrass, and L. T. Wolfe. 1984: An assessment of the impact of pollution on global cloud albedo. Tellus. 36B, 356-366. SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Aerosol Indirect Effect SOEE3410 : Atmosphere and Ocean Climate Change Processes

- Cloud albedo + + + + + + or - ? + + or - ? + + or - ? + or – ? Radiation budget + - Cloud condensation nuclei Global temperature + Sulphate aerosol + Climate feedbacks + SO2 + + or - ? Sea-salt Mean wind DMS + + or - ? + Phytoplankton Abundance & speciation + or – ? DMS Marine ecology + or - ? SOEE3410 : Atmosphere and Ocean Climate Change Processes

Ocean Circulation & Heat Transport Ocean circulation carries about 50% of total equator-to-pole heat flux SOEE3410 : Atmosphere and Ocean Climate Change Processes

Ocean Temperature & Hurricane Activity Tropical cyclones form over distinct regions: SST > 26C latitude > 8 north/south of equator There is no well defined relationship between increasing SST and tropical cyclone frequency (theoretical or observed) A strong correlation between SST and storm intensity is implied by theory Regions of Tropical Cyclone Generation -2C Sea Surface Temperature 35C SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Integrating the energy dissipated by tropical cyclones over the lifetime of the storms reveals a substantial increase in the total energy dissipated annually over the last 55 years. Vmax is the maximum wind speed,  is the duration of the storm. PDI = Power Dissipation index A strong correlation with SST is evident in these results. A measure of total power dissipated annually by tropical cyclones in N-Atlantic and NW-Pacific, and SST (+ offset for comparison). Kerry Emanuel, 2005, Nature, 436/4 August. (doi:10.1038/nature03906) SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Cold trail in N-Atlantic left by Hurricane Fran. SST is ~3-5C cooler in wake of hurricane SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Passage of a tropical cyclone causes a significant drop in sea-surface temperature About 25% due to air-sea sensible and latent heat fluxes 75% due to turbulence generation in upper ocean which deepens the ocean mixed layer, mixing colder water up from below the thermocline. Over the following 1-2 months, the water temperature recovers towards its ‘normal’ value This results in a net heating of the water column which must be balanced by the meridional heat flux in the ocean - + + T SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes The peak meridional heat flux due to ocean circulation has been estimated at 2×1015W The net column heating required to restore surface wakes of tropical cyclones over a year is estimated at (1.40.7)×1015W  hurricanes may drive a significant fraction of the meridional heat flux in the oceans If hurricane activity – intensity or frequency – increases as a result of climate change, then this may result in an increase in the tropically forced thermohaline circulation Estimated that 2C rise in tropical SST could increase the meridional heat flux by up to 30% Increases climate sensitivity in mid-high latitudes (warming effect) and decreases climate sensitivity in tropics (cooling effect) Emanuel, K. 2001, Contribution of tropical cyclones to meridional heat transport by the oceans. JGR 106, D14, 14771-14781 SOEE3410 : Atmosphere and Ocean Climate Change Processes

SOEE3410 : Atmosphere and Ocean Climate Change Processes Summary Feedback processes between different processes within the climate system are complex and often poorly understood Surface-atmosphere interaction involves links between physical, chemical, and biological processes Apparently localised processes can have climate impacts on much larger scales SOEE3410 : Atmosphere and Ocean Climate Change Processes