Focus 1 - Rationale and Goals


The fundamental challenge of this focus was to produce improved, extended, and consistent time series of climate forcings, both natural and anthropogenic. Accurate reconstructions of climate forcings allow climate system models to be used to quantify the spatial and temporal sensitivity of the climate system, and to understand its natural variability (see figure). This allows us to put the present, past, and projected future climate changes in context.

Emphasis was placed on on the last 2 kyr, the Holocene and the Pleistocene, at annual, annual-to-centennial, and glacial-interglacial to sub-millennial timescales, respectively.

Climate forcings are imposed radiative perturbations of the Earth’s energy balance and can be of natural or anthropogenic origin (NRC, 2001).

We distinguished between primary and secondary forcings. Primary climate forcings are those that are externally imposed on the climate system, and do not result from natural feedback processes in the Earth System. Orbital solar insolation, irradiance intensity and volcanic aerosols are therefore classified as primary forcings. Secondary natural climate forcings include those induced by mineral dust, greenhouse gases, land cover, sea ice, continental ice and sea level. They also impact the radiative balance of the atmosphere but are classified as secondary because their level is controlled by feedbacks in the Earth System, and hence depend directly on the climate state itself. Some forcings can be both primary and secondary, such as greenhouse gases, land cover, and aerosols.


- Orbital solar insolation - To tie together the timing of insolation changes and of climatic and environmental responses and to unravel how the geometric, spatial, and seasonal components of orbital insolation changes affected the Earth System.

- Solar irradiance – To improve the documentation and understanding of solar irradiance variations. This necessitates progress in our understanding of irradiance proxies, such as sunspot numbers and cosmogenic signatures in ice cores and tree rings, and that we disentangle solar from non-solar influences. The ultimate goals are to extend the low-frequency solar forcing record back through the entire Holocene, retrieve more detailed and spatially distributed records of cosmogenic isotopes, and interpret them in association with modeling of the mechanisms affecting the isotope concentration in ice and marine sediments.

- Volcanic aerosols – To establish dates, latitude, magnitude and radiative impact of explosive volcanic eruptions through correlation of more ice core records and the development of new tracers, such as the identification of stratospheric eruptions through studies of isotopes in sulfate. The ultimate goal is to extend the detailed record of volcanic forcing through the entire Holocene.

- Mineral dust – To better understand dust loading at a variety of sites—high and low latitudes, continental and oceanic—in order to better constrain the temporal and spatial character of mineral aerosol forcing over glacial-interglacial cycles.

- Greenhouse gases – To improve the record of atmospheric N2O variations and the temporal resolution of the CO2 records. To better constrain the phasing between insolation, greenhouse gas concentrations and climate-environment responses during climate transitions; development of coupled carbon-climate models in the frame of glacial-interglacial changes.

- Land cover – To obtain better records and understanding of regional changes in land use and land cover though vegetation reconstructions and indirect evidence, such as paleofire activity, denudation and soil erosion rates.

- Sea ice – To improve seasonal reconstructions of past changes in sea-ice cover for a better quantification of the albedo and ocean-atmosphere gas exchange of the high-latitude oceans.

- Continental ice and sea level – To improve the reconstruction of the extension, geometry and volume of past ice sheets, and to produce a master sea-level curve for the last glacial-interglacial cycles.





National Research Council, 2001: Climate Change Science: An Analysis of Some Key Questions, National Academy Press: 29.