Sub-Milankovitch Cycles

How to understand volcanism

Volcanic material is found in ice cores etc. so can accurately track the history of past volcanic activity

Impact of volcanism

Ash into stratosphere

Strong winds blow around the world. Therefore is easier if near the equator

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Sulphuric acid droplets can be formed with reactions in the atmosphere → aerosols produced similar to industrial areas → prevents solar radiation reaching the surface → cooling

Magnitude of Volcanic impact

Can have cooling of 0.2-0.3°C

Accounts for 22% of temp variability over last 1000 years

1920-1950 rise in temps can be related to low volcanic activity

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Issue is that these impacts are relatively short time scale

2-3 years

4 years max

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Also no real regular patterns

Tends to amplify changes that are already occurring due to feedbacks

Link between volcanism and glaciation

Big peak in activity between 15-10ka years ago

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Possibly due to deglaciation after the last glacial maximum

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Crust is being released → pressure released → enhanced large scale eruptions

Sunspot Data

Great data for over 400 years

Starting 1610

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Daily observations since 1849

Sunspot Cyle

Short cycle of roughly 11 years

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Cycles can have different outputs and severities

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Reduced sunspot activity → weaker solar wind + less solar output

Sunspot highs/lows

Maunder Minimum - Late C16th

Dalton Minimum - Early C18th

Modern Maximum - Mid C20th

Issue with Sunspot theory

Linked to climate change but recent solar activity has been decreasing despite temps massively increasing

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No net increase in solar activity from 50s onwards whereas temps have been increasing

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Therefore didn’t cause recent global warming

Warming is also at the surface rather than the atmosphere

Thermohaline Ocean Circulation

Love & Walker (2015)

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Transfers heat around the world driven by variations in water temps and density

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Denser saline stuff sinks

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Deep water returns to the south

Colder at bottom

North Atlantic Ocean circulation

Plays a key role in overall circulation

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Warm water transports north

If disrupted then will have major impact on thermohaline circulation

e.g. by freshening it → less saline. From melting ice sheets etc.

Ice Calving Impact

Creates a cap of fresh water preventing or slowing deep water formation

Heat transfer to northern latitudes reduced → millennial scale cooling

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Evidence in ice rafted debris layers

Bond et al,. (1993) - heinrich layers of ice rafted debris - linked to ice calving events and temp fluxes

Dominant modes of North Atlantic

Love & Walker

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Warm - circulation operates as normal and currently

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Cold Interval - circulation more sluggish, confined to shallower depths due to reduced density

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Heinrich Events - circulation turned off. Icebergs and freshwater injection from ice sheet collapse → Antarctic waters filling North Atlantic