I recently completed the second in a Global Renewable News series under the heading Summary for Policymakers whereby Working Group 1 to the Intergovernmental Panel on Climate Change (IPCC) looked at new evidence of climate change (see GRN Volume 5, Issue 11 – March 13 and Issue 13 – April 2). In this third essay, I continue to explain findings that are having a very real effect on our planet and our lives.

C. Understanding the Climate System and its Recent Changes
Combined observations, studies of feedback processes, and model simulations offer up the best results when understanding recent changes in our climate system. The ability of climate models to simulate recent changes need to be evaluated. This requires consideration of the state of all modelled climate system components at the beginning of a session along with the natural and anthropogenic factors used to drive the models. Working in our favour is the fact that longer and more detailed observations plus improved climate models now enable the attribution of human forcing to detected changes in more of the components.

Human influence on the climate system is clear. This is evident from the increasing GHG concentrations in the atmosphere, positive RF, observed warming, and understanding of the climate system.¹

C.1 Evaluation of Climate Models
There’s no doubt that climate models have improved dramatically. They reproduce observed continental-scale surface temperature patterns and trends over many decades, particularly since the mid-20th century when rapid warming has been most evident and the cooling immediately following large volcanic eruptions.

• The long-term climate model simulations show a definite trend in global mean surface temperature from 1951 to 2012. This agrees with the observed trend. It must be noted, however, there are differences between simulated and observed trends over periods as short as 10 to 15 years.
• The observed reduction in surface warming trends over the period 1998 to 2012 as compared to the period 1951 to 2012 is due in roughly equal measure to a reduced trend in RF and a cooling contribution from natural internal variability. This includes a possible redistribution of heat within the ocean. The reduced trend in RF is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. It is, though, difficult to quantify the role of changes in RF in causing the reduced warming trend. Natural internal decadal variability may or may not cause, to a substantial degree, the difference between observations and the simulations; the latter are not expected to reproduce the timing of natural internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing GHGs and other anthropogenic forcing (dominated by the effects of aerosols).
• On regional scales, the confidence in model capability to simulate surface temperature is less than for the larger scales.
• Recently, there has been substantial progress in the assessment of extreme weather and climate events. Simulated global-mean trends in the frequency of extreme warm and cold days and nights over the second half of the 20^th century are generally consistent with observations.
• There has also been improvement in the simulation of continental-scale patterns of precipitation. At regional scales, precipitation is not simulated as well, and the assessment is hampered by observational uncertainties.
• Some important climate phenomena are now better reproduced by models. For example, the statistics of monsoon and El Niño-Southern Oscillation (ENSO) based on multi-model simulations.
• Climate models now include more cloud and aerosol processes, and their interactions but there remains some doubt in the representation and quantification of these processes in such models.
• There is robust evidence that the downward trend in Arctic summer sea ice extent since 1979 is now reproduced by more models, with about one-quarter of the simulations showing a trend as large as, or larger than, the trend in the observations. Most models cover a small downward trend in Antarctic sea ice extent, albeit with large inter-model spread, in contrast to the small upward trend in observations.
• Many models reproduce the observed changes in upper-ocean heat content (0–700 m) from 1961 to 2005 with the multi-model mean time series falling within the range of the available observational estimates for most of the period.
• Climate models that include the carbon cycle (Earth System Models) simulate the global pattern of ocean-atmosphere CO₂ fluxes, with outgassing in the tropics and uptake in the mid and high latitudes. In the majority of these models the sizes of the simulated global land and ocean carbon sinks over the latter part of the 20^th century are within the range of observational estimates.

C.2 Quantification of Climate System Responses
Observational and model studies of temperature change, climate feedbacks, and changes in the Earth’s energy budget together provide confidence in the magnitude of global warming in response to past and future forcing.

• The net feedback from the combined effect of changes in water vapour. Differences between atmospheric and surface warming are extremely likely positive and therefore amplify changes in climate. The net radiative feedback due to all cloud types combined is likely positive. Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds.
• The equilibrium climate sensitivity quantifies the response of the climate system to constant RF on multi-century time scales. It is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO₂ concentration. This assessment reflects improved understanding, the extended temperature record in the atmosphere and ocean, and new estimates of RF.
• The rate and magnitude of global climate change is determined by RF, climate feedbacks, and the storage of energy by the climate system providing strong evidence for our understanding of anthropogenic climate change.
• The transient climate response quantifies the response of the climate system to an increasing RF on a decadal to century timescale. It is defined as the change in global mean surface temperature at the time when the atmospheric CO₂ concentration has doubled but is unlikely greater than three degrees Celsius.
• Various metrics can be used to compare the contributions to climate change of emissions of different substances. The most appropriate metric and time horizon will depend on which aspects of climate change are considered most important to a particular application. No single metric can accurately compare all consequences of different emissions, and all have limitations and uncertainties. The Global Warming Potential is based on the cumulative RF over a particular time horizon, and the Global Temperature Change Potential is based on the change in global mean surface temperature at a chosen point in time.

In the next issue, I will discuss detection and attribution of climate change and move on to future global and regional changes looking at emissions or concentrations of GHGs, aerosols, and other climate drivers. This information is often shown as a scenario of human activities, which are not assessed in these essays. The topics have instead focussed on anthropogenic emissions without studying changes in natural drivers like solar or volcanic forcing or natural emissions such as, CH₄ and N₂O.

I say again:

It’s time to shelve the hubris and try to understand the ramifications of these on-going changes. We need to use our combined intelligence to learn from these findings and save this planet – the very one that has given us the life that we enjoy but are definitely taking advantage of.
 

¹ Stocker, T.F. et al. IPCC, 2013: “Summary for Policymakers.” Climate Change 2013: the Physical Science Basis. Contribution of Working Group 1 to the Fifth Assessment Report (AR5) of the IPCC.