Conceptualising the ocean’s role in the climate system

Whilst we have been attempting to understand changes in ocean circulation due to climate change, it is useful to present a brief sketch of the earth’s climate system. The earth’s climate demonstrates variation on a variety of timescales, scaling from interannual, interdecadal, interannual, millennial and geological scales (Mann, 2007). Thus, the variation of the Earth’s climate can be conceptualised as the product of exogenous (factors independent of anthropogenic change and/or changes in other variables) and endogenous variables (factors affected by anthropogenic change and/or changes in other variables).

In relation to this conceptualisation, ocean circulation can be understood as an endogenous factor in the earth’s climate system.  Given that the current timescale of the Holocene (for purposes of simplicity this term is invoked) is shorter than that of orbital variations (eccentricity, precession and obliquity), termed “Milankovitch pacemakers”, events which occur at “sub-milankovitch periodicity” must be invoked to account for Holocene climate variability. Solar radiation and volcanic eruptions can be understood as examples of exogenous forcing factors and greenhouse gas emissions, EL Nino and Southern Oscillation (ENSO) are examples of endogenous forcing factors.

An interesting point in the debate is that of attribution; Is climate-change natural and /or anthropogenic driven? Crowley (2000) running a model similar to that of the IPCC, found that over the last 1000 years, solar forcing and volcanic forcing explained 41 – 59 % variance. However, when including greenhouse gas emissions and tropospheric aerosols in that scenario, it explained 41-64% of variance.

A question you may ask then is what accounts for the remaining 36 %?

I would argue that most of the uncertainty in the climate change debate is that it is difficult to attribute any degree of climate change conclusively to internal or external factors due to “noise” in the climate system. This can be defined by the fact that multiple possibilities exists for conceptualising the response of the climate system to a specific exogenous or endogenous forcing factors (Maslin and Christensen, 2007) (Figure 1). The climate system may respond directly in response to a variable (Figure 1a), in a delayed fashion (Figure 1b), a muted fashion (Figure 1c) or after passing a threshold (Figure 1d). To complicate this further, bifurcations exist, whereby forcing required to go through a threshold is different to the reverse, implying that once a threshold has crossed, it is difficult to reverse. This can be inferred from previous posts in relation to the impact of meltwater on the deep-water circulation.

Figure 1: Four Possible responses of the global climate system to forcing factors a) Linear b) Muted C) Non-Linear and D) Threshold.

Source: Maslin and Christensen (2007).

Whilst each element of the climate system may respond in variety of ways described above, this is compounded by the fact that each factor in the climate system responds on a different timescale. In this sense, the climate system is anti-essentialist in a way, the nature of climate system respond is multiple and complex. 

This conceptualisation illustrates the complexity of climate change science in particular in response to the question of attribution; detangling the greenhouse signal from that of natural climate variability.