The geologic record shows clearly that the sun is the main climate driver. The Milankovitch multi-millennial orbital cycles in NH insolation are firmly established in the record as are the Schwab and deVries cycles. Other millennial and decadal variations in solar activity are present in the record. TSI is not the only or even the best indicator of solar activity – variations in EUV radiation and the GCR flux (via cloud formation and earth’s albedo) seem to be more important on decadal and centennial scales . Earth’s climate is the result of complex resonances between all these solar cycles with the lunar declination cycles and endogenous earth processes.
At this time the sun has entered a quiet phase with a dramatic drop in solar magnetic field strength since 2004. This suggests the likelihood of a cooling phase on earth with Solar Cycles 21, 22 ,23 equivalent to Solar Cycles 2,3,4, and the delayed Cycle 24 comparable with Cycle 5 so that a Dalton type minimum is probable.
It is now clear that the patterns of the earth’s ocean and atmospheric current systems provide the most immediate guide to the current state of the climate when climate is considered as the result of the complex processes discussed above and these systems provide the best clues as to l developments over the next 20 – 30 years. Beyond that time span predictions are currently of little practical value. The small effect of anthropogenic CO2 cannot be calculated with any useful accuracy until we can better quantify the effect of the natural variations.
The IPCC CO2 -anthropogenic warming projections are based on models. AR4 WG1 section 8.6.4 deals with the reliability of the projections.This IPCC science section on models concludes
"Moreover it is not yet clear which tests are critical for constraining the future projections,consequently a set of model metrics that might be used to narrow the range of plausible climate change feedbacks and climate sensitivity has yet to be developed"
What could be clearer. The IPCC itself says that we dont even know what metrics to put into the models to test their reliability.- ie we don't know what future temperatures will be and we can't calculate the climate sensitivity to CO2.This also begs a further question of what mere assumptions went into the "plausible" models to be tested anyway.
Of particular note in climate forecasting is the negative phase of the PDO which began perhaps five years ago and may well last for another 25years. This suggests that La Ninas will be more frequent than El Ninos during this time span. A general earth cooling is thus more likely as was the case from 1940 to 1970 when similar conditions prevailed. Concurrent changes in the Arctic Oscillation suggest a pattern of meridional atmospheric flow will be more common than the more latitudinal flows of warmer periods.
Policymakers may wish to note the following possible effects on earth’s climate for the next 20 – 30 years. A cooler world with lower SSTs usually means a dryer world. Thus droughts will be more likely in for example east Africa with possible monsoon failures in India. In California the PDO will mean less rainfall with more forest fires in the south. However in the Cascades and Northern Sierras snowpack could increase since more of the rain could occur as snow. Northern Hemisphere growing seasons will be shorter with occasional early and late frosts and drought in the US corn belt and in Asia repeats of the harsh Mongolian and Chinese winters of 2009 – 10 . In Europe cold snowy winters and cool cloudy summers will be more frequent .
There will be a steeper temperature gradient from the tropics to the poles so that violent thunderstorms with associated flooding and tornadoes will be more frequent in the USA, At the same time the jet stream will swing more sharply North – South thus local weather in the Northern hemisphere in particular will be generally more variable with occasional more northerly heat waves and more southerly unusually cold snaps. In the USA hurricanes may strike the east coast with greater frequency in summer and storm related blizzards more common in winter.
The southern continents will be generally cooler with more frequent droughts and frost and snowin winter,
Arctic and Antarctic sea ice may react differentially to an average global cooling. We might expect sea ice to increase in the Antarctic but in the NH the Arctic Oscillation while bringing cooler temperatures further south may also occasionaly bring warmer air into the Arctic with possible relative loss of sea ice in that areaduring those years.
The most general advice is that world food production will be subject to occasional serious severe restriction because of cold and drought. The use of food crops for biofuels should be abandoned and stockpiles built up for possible lean times ahead.. Northern cities and transportation systems should prepare for more frequent heavy snow and ice storms.
There is no threat from the burning of fossil fuels for the forseeable future, indeed an increase in CO2 would positively help in feeding the burgeoning population.
For the next 20 years climate science should be devoted to improving and enlarging the entire climate data base in particular with regard to solar data of all kinds. No climate model runs should be made until 2025 by which time the inputs will hopefully be more relevant to the real world.