Frequently asked questions
FAQ 5.1: Is the sun a major driver of recent climate change?
Solar variability—including the 11-year sunspot cycle and longer-term trends in solar irradiance—is one important factor in determining global average surface temperatures. However, direct measurements of total solar irradiance (TSI) since 1978 show that global temperature anomalies cannot be explained by this solar variability. The 11-year sunspot cycle and longer trends tend to result in variations in TSI of only about 0.1%, which is enough to affect global average surface temperature by about 0.1ºC. Recent warming has been greater than that (~0.8ºC), and TSI has actually decreased slightly since the late 1970s. The full magnitude of recent warming trends can only be explained with anthropogenic changes, as illustrated in the figure to the right.
FAQ 5.1, Figure 1: Global temperature anomalies from 1870 to 2010 relative to a 1961-1990 baseline. (a) Observed temperature anomalies (black line) and modeled anomalies from sum of natural and anthropogenic factors (red line.) (b) Temperature anomaly due to solar forcing. (c) Temperature anomaly due to volcanic eruptions. (d) Temperature anomaly due to internal variability, here related to the El Niño-Southern Oscillation. (e) Temperature anomaly due to anthropogenic forcing--warming from greenhouse gases and cooling from aerosols.
FAQ 5.2: How unusual is the current sea level rate of change?
“The rate of mean global sea level change—averaging 1.7 ± 0.2 mm yr–1 for the entire 20th century and between 2.8 and 3.6 mm yr–1 since 1993 (Chapter 13)—is unusual in the context of centennial-scale variations of the last two millennia. However, much more rapid rates of sea level change occurred during past periods of rapid ice sheet disintegration, such as transitions between glacial and interglacial periods. Exceptional tectonic effects can also drive very rapid local sea level changes, with local rates exceeding the current global rates of change." (IPCC AR5 Chapter 5, FAQ 5.2)
The figure below summarizes rates of sea level change for both past episodes of rapid change and the modern. See the section on past changes in sea level or the summary of Chapter 13 (Sea level change) for more information.
Other FAQs not included in Chapter 5:
When was the Earth last as warm as present? What is the best analogue for future warming?
Unfortunately, this is not as straighforward a question as it seems. There are multiple problem: unreliable data/model output, regional differences (i.e., the Arctic may be warmer now than since the Last Interglacial, but the tropics may have been warmer than present during the early Holocene), problems with time scale and seasonality, etc. The IPCC cautiously states that recent warming in the Northern Hemisphere (data is insufficient for the Southern Hemisphere) has likely resulted in the warmest 30-year period in the last 1400 years. However, the Medieval Climate Anomaly (950-1250) may have been warmer than the 20th Century in some areas. Others have argued that modern warmth exceeds that since the early- to mid-Holcene, and some recent work points to unprecedented warming since the Last Interglacial (Miller et al., 2013). The most robust, reliable estimates are what is published in AR5, however, and further measurements will be needed to validate more substantial claims.
What should I believe, the data or the models?
Both data and models come with a fair share of limitations. Paleoclimate data are based on proxies, which are indirect climate indicators. Sometimes, it is unclear whether a certain proxy's relationship with a climate variable has held constant through time. Other times, it is unclear exactly what climate parameter a proxy represents (is it summertime sea surface temperature? Annual air temperature? A combination of temperature and precipitation?). Most proxies require calibration (i.e., a mathematical relationship that takes a measured value, like an isotopic ratio, to a useful climatic metric, like temperature), and calibrations are imperfect and are not necessarily constant for the entire Earth or for all of Earth's history. Finally, the collection of paleoclimate data is expensive and time-consuming, so the spatial coverage of paleoclimate records is far from complete. Interpretations of paleoclimate in areas that don't have local records is just an assumption that regional patterns have held through time. Despite all of these caveats, paleoclimate data provide invaluable insight into the functioning of Earth's climate system in the past.
Models come with their own limitations. Although current climate models include highly complex mathematical representations of the physics of the climate system, they are inherently limited in representing a chaotic system. A significant problem in modeling the past is that the boundary conditions--factors like how continents were arranged, what the vegetation cover was like, and what the atmospheric composition was--are hard to know exactly. Running computer models is also extremely expensive and time-consuming, so running fully coupled models for periods millions of years long is impossible. Instead, snap-shots of past climate are taken to try and understand past conditions. Still, models are extremely useful and have improved significantly in reproducing data-driven estimates of past climate conditions.
In some cases, data are probably more reliable, and in other cases, models may do a better job. Models have generally underestimated polar amplification (e.g., Masson-Delmotte et al., 2005), although they are improving in this respect. Further work in both the data and modeling realm are needed to make more robust data-model comparisons.
FAQ 5.2, Figure 1: Average rates of sea-level change (measured in millimeters per year) for past and current eras of climate change. Rates of sea level rise due to anthropogenic factors (20th Century and Satellite Altimetry Era [1993-2012]) are significant but still lower than periods in the geologic past when sea level rose dramatically due to widespread melting of ice sheets.