Hi All
The effects of reduced precipitation from stratospheric aerosols
identified by Farraro are there with marine cloud brightening but it
also has others. Making the sea cooler than the land makes stronger
monsoons to move more water to land. Making stronger winds over the sea
makes for more breaking waves and so more evaporation. Making smaller
drops in clouds over the sea reduces rainfall over the sea to leave more
to fall further inland.
Bala and Caldeira in DOI 10.1007/s00382-010-0868-1 say that the overall
result of the conflicting effects could lead to increased river run off
with a contribution from reduced evaporation ashore.
Nobody cares much about precipitation over sea except for people living
on small islands who might be more worried about rising sea levels. It
is possible to use the power in sea waves to do carbon-free desalination.
Stephen
Emeritus Professor of Engineering Design. School of Engineering,
University of Edinburgh, Mayfield Road, Edinburgh EH9 3DW, Scotland
[email protected], Tel +44 (0)131 650 5704, Cell 07795 203 195,
WWW.homepages.ed.ac.uk/shs, YouTube Jamie Taylor Power for Change
On 06/03/2016 12:20, Andrew Lockley wrote:
https://angusferraro.wordpress.com/2016/03/05/can-stratospheric-aerosols-directly-affect-global-precipitation/
Can stratospheric aerosols directly affect global precipitation?
What is the effect of stratospheric aerosol geoengineering on global
precipitation? If we were to inject sulphate aerosol into the
stratosphere it would reflect some sunlight and cool the Earth, but
the atmosphere’s CO2 levels would remain high. This is important,
because CO2 actually has an effect on precipitation even when it
doesn’t affect surface temperature. In a recent paper with a summer
student, I’ve shown the aerosols can contribute a similar effect.
Three climate models (CanESM2, HadGEM2-ES, MPI-ESM-LR) did simulations
of the future with and without geoengineering. The simulations with
stratospheric aerosols (G3 and G4) show greater
temperature-independent precipitation reductions than the simulations
without them (RCP4.5 and G3S).
Precipitation as energy flow
Precipitation transfers energy from the Earth’s surface to its
atmosphere. It takes energy to evaporate water from the surface. Just
as evaporation of sweat from your skin cools you off by taking up heat
from your skin, evaporation from the Earth’s surface cools it through
energy transfer. Precipitation occurs when this water condenses out in
the atmosphere. Condensation releases the heat energy stored when the
water evaporated, warming the atmosphere. Globally, precipitation
transfers about 78 Watts per square metreof energy from the surface to
the atmosphere. Multiplying that by global surface area that’s a total
energy transfer of about 40 petajoules (that’s 40 with 15 zeros after
it) of energy every second! To put that in a bit of context, it’s
about 40% of the amount of energy the Sun transfers to the Earth’s
surface.
If precipitation changes, that’s the same as saying the atmospheric
energy balance changes. If we warm the atmosphere up, it is able to
radiate more energy (following the Stefan-Boltzmann law). To balance
that, more energy needs to go into the atmosphere. This happens
through precipitation changes.
Direct effects of gases on precipitation
Now imagine we change the amount of CO2 in the atmosphere. This
decreases the amount of energy the atmosphere emits to space, meaning
the atmosphere has more energy coming in than out. To restore balance
the atmospheric heating from precipitation goes down. This means that
the global precipitation response to global warming from increasing
CO2 has two opposing components: a temperature-independent effect of
the CO2, which decreases precipitation, and a temperature-dependent
effect which arises from the warming the CO2 subsequently causes. In
the long run the temperature-dependent effect is larger. Global
warming will increase global precipitation – although there could be
local increases or decreases.
But what happens if we do geoengineering? Say we get rid of the
temperature-dependent part using aerosols to reduce incoming solar
radiation. The temperature-independent effect of CO2 remains and
global precipitation will go down.
Detecting the effect of stratospheric aerosol
CO2 isn’t the only thing that has a temperature-independent effect.
Any substance that modifies the energy balance of the atmosphere has
one. In our newstudy, we ask whether stratospheric sulphate aerosol
has a detectable effect on global precipitation. Theoretically it
makes sense, but it is difficult to detect because usually there are
temperature-dependent effects obscuring it.
We used a common method to remove the temperature-dependent effect. We
calculated the precipitation change for a given surface temperature
change from a separate simulation, then used this to remove the
temperature-dependent effect in climate model simulations of the
future. We did this for future scenarios with and without geoengineering.
As expected, we found a temperature-independent influence which
reduced precipitation. Importantly, this effect was bigger when
geoengineering aerosols were present in the stratosphere. This was
detectable in three different climate models. The figure above shows
this. The non-geoengineered ‘RCP4.5’ simulation shows a precipitation
decline when the temperature effect is removed. This comes mainly from
the CO2. The ‘G3’ and ‘G4’ geoengineering simulations (blue and green
lines) have an even greater decline. The aerosol is acting to decrease
precipitation further.
How does aerosol affect precipitation?
The temperature-independent effect wasn’t present when geoengineering
was done by ‘dimming the Sun’. The ‘G3S’ simulation (orange lines in
the figure) does this, and it has a similar precipitation change to
RCP4.5. So what causes the precipitation reduction when stratospheric
aerosols are used? We calculated the effect of the aerosol on the
energy budget of the troposphere (where the precipitation occurs). We
separated this in two: the aerosol itself, and the stratospheric
warming that occurs because of the effect of the aerosol on the
stratosphere’s energy budget.
Black bars show the temperature-independent precipitation changes
simulated by the models. Orange bars show our calculation of the
effect of the stratospheric warming. Green bars show our calculation
of effect of the aerosol itself. Grey bars show our calculation of the
total effect, which is very close to the actual simulated result.
We found the main effect was from the aerosol itself. The aerosol’s
main effect is to reduce incoming solar radiation and cool the
surface. But we showed it also interferes a little with the radiation
escaping to space, and this alters the energy balance of the
troposphere. The precipitation has to respond to these energy balance
changes.
This effect is not huge. We had to use many model simulations of the
21st Century to detect it above the ‘noise’ of internal variability.
In the real world we only have one ‘simulation’, so this implies the
temperature-independent effect of stratospheric aerosol on
precipitation would not be detectable in real-world moderate
geoengineering scenario. This also means climate model simulations not
including the effects of the aerosol could capture much of the effects
of geoengineering on the global hydrological cycle.
This effect could be more important under certain circumstances. If
geoengineering was more extreme, with more aerosol injected for
longer, precipitation would decrease more. But, based on these
results, the main effect of geoengineering on precipitation is that
the temperature-dependent changes are minimised. This means the
temperature-independent effect of increasing CO2 concentrations is
unmasked, reducing precipitation.
Take a look at the paper for more details – it’s open access!
Ferraro, A. J., & Griffiths, H. G. (2016). Quantifying the
temperature-independent effect of stratospheric aerosol geoengineering
on global-mean precipitation in a multi- model ensemble. Environmental
Research Letters, 11, 034012. doi:10.1088/1748-9326/11/3/034012.
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