Last IPCC Report. What we certainly know about climate change.

Cover of the last IPCC report 'Climate Change 2013: The Physical basis'. Source: IPCC

On Friday, the IPCC published its last report. Probably, you have listened or read something about it during these days. What is the IPCC? and (2) What is this report about?

The IPCC is the Intergovernmental Panel on Climate Change (IPCC). It was established in 1988 by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and socio-economic impacts. How? The IPCC reviews and assesses the most recent scientific, technical and socio-economic information produced worldwide relevant to the understanding of climate change. Thousands of scientists from all over the world contribute to the work of the IPCC, bringing their expertise in the many different disciplines necessary to produce a comprehensive assessment of climate change on a voluntary basis. For the preparation of the last report on ‘Climate Change. The Physical Science Basis’, a total of 259 Lead Authors and 50 Review Editors from 39 countries and more than 600 Contributing Authors from 32 countries contributed. You can download the report here.

According to the information published on this last report, I want to resume some of things that we 'certainly know' about climate change based on what the last decades of observations tell us. Why do I remark the fact that ‘we  certainly know’? Because the report is cautiously written. It means that consensus is necessary among the scientists that contribute to the report when the degree of certainty in key findings is expressed from very low to very high and from exceptionally unlikely to virtually certain. Thus, findings considered virtually certain are supported by data, theory, models, etc. and the scientific consensus. Those are some of these findings.

• ‘Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.’

•  Atmosphere: ‘Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.’  (Fig. 1)

Fig 1. (a) Annual and (b) decadal average surface temperature anomaly. 
In addition to robust multi-decadal warming, we observe here that global mean surface temperature exhibits substantial decadal and interannual variability. The fact that natural variability can make for short periods temperature trends negative (cooling) doesn't blur the long term trend. This last must always be calculated based on long term records and not in short term records. Source: IPCC report

• Ocean: ‘Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010. More than 60% of the net energy increase in the climate system is stored in the upper ocean (0–700 m) during the relatively well-sampled 40-year period from 1971 to 2010, and about 30% is stored in the ocean below 700 m.’
• Cryosphere (Earth's surface where water is in solid (ice, snow) form): ‘Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent (high confidence).’
• Sea level: 'The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence). Since the early 1970s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global mean sea level rise (high confidence).’
• Carbon and other biogeochemical cycles: The atmospheric concentrations of the greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) have all increased since 1750 DuE To HuMAn ACTiViTY, reaching levels unprecedented in at least the last 800,000 years. CO₂ concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. -Here, some numbers that perhaps will surprise you: From 1750 to 2011, CO₂ emissions from fossil fuel combustion and cement production have released 365 [335 to 395] GtC to the atmosphere, while deforestation and other land use change are estimated to have released 180 [100 to 260] GtC. Thus, the 'secondary role' of deforestation and other land use is not negligible.- The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.

Fig. 2. (a) Atmospheric concentrations of carbon dioxide (CO2) from Mauna Loa (19°32′N, 155°34′W – red) (I told you more about this time-series here) and South Pole (89°59′S, 24°48′W – black) since 1958; (b) partial pressure of dissolved CO2 at the ocean surface (blue curves) and in situ pH (green curves), a measure of the acidity of ocean water (click here for more information about ocean acification). Measurements are from three stations from the Atlantic (29°10′N, 15°30′W – dark blue/dark green; 31°40′N, 64°10′W – blue/green) and the Pacific Oceans (22°45′N, 158°00′W − light blue/light green). Source: IPCC report.

• Drivers of climate change. Almost all of the energy which affects Earth's weather is received as radiant energy from the Sun. The planet and its atmosphere absorb and reflect some of the energy at different length waves, and the balance between absorbed and radiated energy determines the Earth’s average temperature. Natural and anthropogenic factors as the intensity of solar energy, reflectivity of clouds or gases, and absorption by various greenhouse gases alter the radiation balance, becoming a climate change driver. Why? Because any change in these factors (e.g.: increase of greenhouse gases concentration) result in a radiative forcing (e..: more energy is absorved, and so less energy leaves) causing a new balance to be reached. A positive forcing (more incoming energy) warms the system, while negative forcing (more outgoing energy) cools it. 
  Now, that you know that you can understand more easily the IPCC finding: 
  
  ‘Total radiative forcing is positive, and has led to an uptake of energy by the climate system (warming). The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO₂ since 1750.’
  
  Conclusion:

‘HuMAn INFLuENCe on the climate system is CLeAR. This is EViDeNT from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system’.

Questions???

 And... one moment before you stop reading, although I haven't mentioned anything, the report also includes information about the changes that we can expect in our climate for the end of this century. I have preferred only focus in what we have observed until know because: first, I'm an observational oceanographer; and, second, you can read an excellent resume of that information in RealClimate.