1. A positive feedback does not mean that something good is happening in the Earth system. Rather, it means that a change is being amplified by other Earth-system processes that were triggered by the change. From a human perspective, having a change amplified could be good or bad.
2. When ice is present, relatively small changes in temperature can trigger albedo feedbacks that amplify the change. If no ice is present, a small increase in temperature might not have much of an impact. But if ice is present, that small increase could be enough to cause the ice to melt, and decrease Earth’s albedo. If ice is created when the climate cools, further cooling can be triggered if the ice increases Earth’s albedo significantly.
3. We use 65° for estimating the glaciation potential of orbital variations because glaciers are most likely to form at high latitudes, where temperatures are cooler, and insolation less direct. We use 65° N rather than 65° S because for more than 50 million years the continents have been concentrated in the northern hemisphere. We use July instead of January, because for ice to form and remain year-round, it is necessary to have cool summers.
4. Gondwana was situated over the south pole for much of the Paleozoic. Not only was the land subject to temperatures cooler over all, and less direct insolation for part of the year, but the large size of the continent limited the extent to which heat from the ocean could reach the interior of the continent. The development of ice on Gondwana further cooled the planet through albedo feedback.
5. If the major currents in the oceans were to slow down or stop, the tropics would get hotter and the high-latitude areas would get colder, leading to expansion of glaciers and sea ice. The various feedbacks (e.g., higher albedo because of increased ice cover) would result in a cooler climate over all.
6. From a climate perspective, the two important volcanic gases are SO2 and CO2. SO2 is converted to aerosols which block sunlight and can lead to short-term cooling (lasting years). CO2 can lead to warming, but only in situations where there is an elevated level of volcanism over the long term.
7. Greenhouse gases vibrate at frequencies that are similar to those of infrared radiation. When infrared radiation hits a greenhouse gas molecule, the molecule’s vibrational energy is enhanced and the radiation energy is converted into heat, which is trapped within the atmosphere.
8. Proxy data can provide information about climate in the distant past, for times during which there were no direct measurements. As new techniques are developed, analyses of proxy data can improve, whereas direct measurements will have experimental errors related to the instruments used to make them at the time. A disadvantage of proxy data is that we don’t always have a detailed timeline to go with the data, and may need to rely on other lines of evidence to understand when the proxy data should apply. Special equipment or techniques may be needed to get proxy data from geological materials. It is also necessary to ensure that geological processes have not altered the materials in some way that makes the results of analyses unreliable.
9. Real-life measurements are used to set up the model, and to test that it can provide realistic results. Many different models may be used to study the same scenario, and the results compared. This helps scientists to compare the results of representing the Earth system in different ways. An estimate of uncertainty is provided so scientists can decide whether or not the range of uncertainty is too big, given the size of the Earth-system change being studied.
10. Data from ice cores and direct measurements show that atmospheric CO2 levels began to rise very rapidly at the same time that humans began using fossil fuels extensively to power activities like manufacturing and transportation. At the same time, the stable carbon-isotope composition of atmospheric CO2 began to decrease at a rate that is consistent with the stable carbon-isotope fingerprint of plant-derived organic matter. Also simultaneously, the radiocarbon age of atmospheric CO2 began to fall, consistent with carbon from very ancient plant sources being added to the atmosphere. Aside from the timing of geochemical changes matching the onset of the industrial era, there are no other sources of carbon that can account for the geochemical fingerprint in the atmosphere.
11. The reason that relatively small anthropogenic flows of carbon into the carbon cycle can have a big impact is that they aren’t being balanced by natural processes that can remove the extra carbon. The result is that carbon from anthropogenic activities is accumulating in the atmosphere and ocean.
12. In the past, Earth’s climate has been both much colder and much hotter than today. However, present day temperatures are significantly different compared to global average temperatures over the past 1000 years, and stand out in the data from the past 800,000 years. Atmospheric CO2 levels are much higher than at any time during the past 800,000 years, and on that timescale, the increase is almost instantaneous. Therefore, the magnitude of present-day change in the Earth-system stands out clearly from background conditions predating the appearance of anatomically modern humans. It is also happening much faster than even the most rapid carbon-release event that we know of in the rock record.
13. Climate warming happens more rapidly at high latitudes more than low latitudes, decreasing the temperature difference between them. The decreased difference allows atmospheric circulation patterns to “stall out” over a location for longer than usual. What would normally be a short heatwave lasts much longer, and a precipitation event that would normally move elsewhere after a short time is now halted in place for weeks.
14. See Figure 16.44. The power of tropical storms depends on the warmth of seawater. Data show that during particularly warm intervals, tropical storms are stronger. Data also show that the strength of storms over all is increasing as sea surface temperatures rise over time.
15. Frozen soils provide a solid foundation for buildings in northern communities. Melting of permafrost means the soil beneath buildings is weakened, and buildings can suffer structural damage. Thawing means that hillsides are more prone to mass wasting, especially terrain along coasts. In terms of the climate system, thawing of permafrost allows micro-organisms to decompose organic matter within previously-frozen sediments, releasing additional CO2 and CH4, and contributing to a positive feedback.