Exploring signatures of natural forcings on Last Millennium temperature: certainties and uncertainties

Abstract:

Large-scale temperature during the Last Millennium has been mainly influenced by naturally varying forcing: orbital forcing, volcanic and solar forcing. While the variation of the orbital parameters is precisely known, volcanic and solar forcing are subject to significant uncertainties. The exact knowledge of the orbital parameters allows us to pin down a precise signature of orbital forcing on the climate, using orbital data, climate model output and proxy reconstructions. We find that seasonal orbital forcing plays a major role in long-term trends of northern hemisphere climate model output. While proxy reconstructions indicate a similar seasonal trend pattern, the detection and attribution of orbital forcing in reconstructions is limited by uncertainties and availability of proxy data.

The additional uncertainties in solar and volcanic forcing data make it difficult to pin down the climate response recorded by proxy data. To understand the role of forcing uncertainty, we investigate effects of uncertainties on simulated global temperatures, using an ensemble of volcanic forcing reconstructions with varying eruption magnitude and time, along with different reconstructions of solar variability. Our ultimate aim is to quantify the role of forcing uncertainty in comparison to other uncertainties, and to find if certain forcing realisations can improve the agreement between model simulations and proxy reconstructions.