Past Radiative Forcing
What are the causes of past climate change, and how important are each of the contributing factors? Causes of climate change are referred to as "forcings," and typically include only factors outside of the climate system itself. In order to account for paleoclimate fluctuations, it is important to consider both (1) external forcings and (2) changes in atmospheric composition (like greenhouse gas [GHG] concentrations). The magnitude of the climatic impact of these various factors is quantified in terms of radiative forcing.
Orbital forcings change in regular, cyclical patterns on timescales of tens to hundreds of thousands of years and are the primary driver of glacial-interglacial cycles. Orbital variations are also responsible for the millennial-scale climatic variations seen during the Holocene (like the warmth of the Mid-Holocene). Researchers are still trying to determine exactly how orbital variations result in the magnitude of temperature change observed between glacial and interglacial periods, along with significant fluctuations in ice sheets.
Total solar irradiance (TSI) fluctuates according to a regular 11-year solar cycle and periodic grand solar minima (like the Maunder Minimum of 1675-1715), as well as on longer timescales. TSI reconstructions from the pre-satellite era depend on sun spots and cosmogenic radionuclides like Beryllium-10. The most recent reconstructions and models indicate that the difference in TSI between the late 20th century and the Maunder Minimum is <0.1%; thus, solar forcing is important but not enough to explain larger climate changes such as modern warming. Figure 5.1(b) below shows several TSI reconstructions for the last 1000 years.
Volcanic activity has a transient cooling effect due to injection of atmospheric sulphate aerosols (very tiny suspended liquid or solid particles), which block incoming solar radiation. Prehistoric volcanic activity can be reconstructed using sulphate deposits in polar ice sheets. As is shown in Figure 5.1(a), volcanic activity (identifiable by the spikes in the plot) results in a strongly negative radiative forcing (and thus cooling), but the effect is short-lived, as the radiative forcing curve returns to zero quickly after a volcanic eruption. Volcanic forcing is important on very short time scales, but less so for climate trends over centuries or longer.
Greenhouse gases (GHGs) cause warming of the Earth's surface. Air bubbles trapped in polar ice sheets allow for direct measurement of ancient atmospheric composition for the last 800,000 years. These measurements indicate that modern GHG concentrations are unprecedented in the last 800,000 years, during which time atmospheric CO2 fluctuated between 180 and 300 parts per million (ppm) (2013: 396.5 ppm [co2now.org]). The current rate of increase of anthropogenic GHGs is also unprecedented at least in the last 22,000 years, and probably the past 800,000 years.
Indirect measurements, using proxies such as Boron isotopes from marine plankton, leaf stomata counts, and paleosoil properties, allow for estimation of CO2 beyond the ice core record. Figure 5.2 is a compilation of CO2 estimates from various proxy records for the last 65 million years. Between 65 and 23 million years ago, CO2 was relatively high—between 300 and 1500 ppm. Since 23 million years ago, CO2 has remained mostly around pre-industrial levels with some exceptions, including the Mid-Pliocene Warm Period.
Figure 5.1 (a) Two records of volcanic forcings (in watts per meter squared) for the last 1000 years, derived from sulphate found in ice cores. Note that volcanic eruptions resulted in negative radiative forcing (i.e., cooling), but that their effect is short-lived. (b) Several records of TSI for the last 1000 years, derived from Beryllium-10 isotopes and sunspots.
Figure 5.2 Top: Reconstructions of Southern Ocean dust accumulation (from Antarctic ice cores), global sea level (from oxygen isotopes), tropical sea-surface temperature (from geochemical measures in marine sediment cores), and atmospheric CO2 (from Antarctic ice cores and marine sediment cores) for the last 3.5 million years. Bottom: Atmospheric CO2 reconstructed for the last 65 million years, based on various marine and terrestrial proxies.