The tremendous complexity combined with the short-term and local variability of climate make accurately forecasting global climate change one of the most challenging and daunting tasks ever undertaken by the scientific community. Nonetheless, spurred on by growing number of signs that significant climate changes were under way, climate scientists from around the world assembled by UN Intergovernmental Panel on Climate Change (IPCC) have been attempting to do just that since the IPCC's creation in 1988 by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO), four years prior to establishment of an international agreement on climate change mitigation and adaptation, the United Nations Framework Convention on Climate Change (UNFCCC).
The IPCC's fourth global climate assessment report (AR4) was published in 2007. The fifth (AR5) is due to be finalized in 2014, putting “greater emphasis on assessing the socioeconomic aspects of climate change and implications for sustainable development, risk management and the framing of a response through both adaptation and mitigation.”
Now just past the midway point for the 1990-2030 period over which the IPCC issued its initial predictions for global temperature as part of its First Assessment Report (AR 1), two researchers decided it was an opportune time to evaluate their accuracy and performance. The results: the increase in global mean temperature predicted for the period 1990-2010 have turned out to be somewhat surprisingly and unexpectedly accurate.
Actual vs. Predicted Increases in Mean Global Temperature
As report authors David J. Frame of New Zealand's Victoria University and Daithi A. Stone of the Lawrence Livermore Berkeley National Laboratory note in their paper published in Nature Climate Change, IPCC climate change scientists have now been working on the problem of forecasting global mean temperature “for a period comparable to the prediction and the time scales over which climate is expected to respond to these types of external forcing,” a reference to the effects human carbon and greenhouse gas emissions are intensifying the Greenhouse Gas Effect.
Continued work over the course of ensuing time establishes a baseline of actual historical global mean temperature to be recorded that addresses a key statistical weakness in evaluating the accuracy of predictions of long-term climate change.
Based on their models, IPCC scientists in AR 1 came up with a predicted increase in global mean of 0.7°-1.5° C (1.26°-2.7° F) between 1990-2030, with a best estimate of an increase of 1.1° C (1.98° F). Following a linear trend, this corresponds to a best estimate of global warming of 0.55° C (0.99° F) from 1990-2010.
Frame and Stone collected official statistical data used to calculate global mean temperature and annual changes from agencies around the world for the period 1990-2010. Adding an adjustment to account for natural climate variability that weren't incorporated in the IPCC's initial 1990 climate forecast they “found the results fit almost perfectly with the predictions made 22 years ago,” according to a PhysOrg report.
The researchers said the accuracy of the results is remarkable in light of the actual course of climate events, events that could not have been foreseen, such as the eruption of Mt. Pinatubo, reduction of GHG emissions in Russia or the rise in emissions from China's ongoing rapid industrialization.
“Even though the climate research community clearly has much work to do to improve regional climate predictability—the most relevant scales for most impacts—it seems highly likely that even in 1990 we understood the climate system well enough to make credible statements about how its aggregate properties would change on timescales out to a couple of decades, even in the presence of considerable uncertainty surrounding the exact forcing trajectory,” the authors write.
“Exact forcing trajectory” refers to the uncertainty of human greenhouse gas emissions over the period.
“As is always the case in science, we cannot know for certain that the 1990 prediction was accurate for the right reasons, but given the apparent absence of any credible alternative theories and the robustness of the prediction, this evaluation strongly supports the contention that the climate is responding to enhanced levels of GHGs in accordance with historical expectations,” they conclude.
Troubling in terms of efforts to reduce human carbon and GHG emissions, however, is their finding on the degree to which the stock of GHGs already accumulated in the atmosphere overwhelms the flow of emissions year to year.
“Most remarkably, if anthropogenic forcings had been held at 1989 levels over the past two decades the resulting 0.10°-0.48° C trend would still have been consistent with the observed trend and not with the zero trend: as climate predicatability comes from the forcing undefined, it is governed primarily by the accumulating stock of GHGs, that is, concentrations, and is relatively insensitive to short-term details associated with the flow of emissions.”
*Graphic credit: UN IPCC, AR 4