WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.870 Unit C C 17: Earth and atmospheric science. 00:00:10.533 --> 00:00:15.658 The early atmosphere developed substantially from gases released by volcanic activity. 00:00:15.808 --> 00:00:24.251 The GCSE model describes much carbon dioxide, water vapour, little or no free oxygen and smaller amounts of other gases. 00:00:26.933 --> 00:00:31.429 Its exact composition and changes over billions of years are uncertain. 00:00:31.579 --> 00:00:39.909 There are no direct measurements of the earliest atmosphere; evidence comes from rocks, fossils, volcanic gases and comparisons with other planets. 00:00:42.567 --> 00:00:50.384 Venus and Mars have C O 2-rich atmospheres that provide a useful comparison, but they do not prove Earth followed an identical history. 00:00:50.534 --> 00:00:53.807 Saturn is not the intended rocky-planet comparison. 00:00:56.467 --> 00:01:01.189 As Earth cooled, water vapour condensed and rain contributed to forming oceans. 00:01:01.339 --> 00:01:06.213 Carbon dioxide dissolved in the oceans, reducing atmospheric C O 2. 00:01:08.867 --> 00:01:15.777 Carbon was later stored in carbonate sediments, shells and rocks and in organic material that could form fossil fuels. 00:01:15.927 --> 00:01:19.489 These processes helped change the atmospheric carbon balance. 00:01:19.639 --> 00:01:25.143 A simplified process sequence, not a precisely dated or complete atmospheric history. 00:01:29.800 --> 00:01:34.942 Rock chemistry can indicate oxidation conditions and changes in free oxygen. 00:01:35.092 --> 00:01:43.582 A rock containing chemically bound oxygen is not evidence that there was already much O 2 gas; early rocks were not universally oxygen-free. 00:01:46.233 --> 00:01:52.843 Photosynthetic microorganisms, including cyanobacteria, used C O 2 and released oxygen. 00:01:52.993 --> 00:02:01.182 Photosynthesis and oxygen reactions with rocks and oceans contributed to a long, complex rise in atmospheric oxygen. 00:02:03.833 --> 00:02:10.231 A major rise in free oxygen occurred around 2.4 billion years ago, with further changes later. 00:02:10.381 --> 00:02:15.499 Do not picture today's 21 percent oxygen appearing instantly at that time. 00:02:18.167 --> 00:02:27.072 For GCSE revision, dry air is approximately 78 percent nitrogen, 21 percent oxygen and 1 percent other gases. 00:02:27.222 --> 00:02:30.923 The “dry” qualification excludes variable water vapour. 00:02:31.073 --> 00:02:38.217 Nitrogen 78 percent, oxygen 21 percent, other gases about 1 percent; water vapour varies. 00:02:42.900 --> 00:02:53.545 Argon contributes most of the remaining roughly 1 percent; carbon dioxide is around 0.04 percent and its measured concentration changes over time and location. 00:02:53.695 --> 00:02:56.137 Other gases occur in smaller amounts. 00:02:58.800 --> 00:03:06.511 Water-vapour content varies with conditions and location, so percentages for moist air differ from a simplified dry-air composition. 00:03:09.167 --> 00:03:16.003 Oxygen supports combustion and is tested using a glowing splint: it relights in a suitable oxygen sample. 00:03:16.153 --> 00:03:19.054 A lighted-splint pop instead tests hydrogen. 00:03:21.733 --> 00:03:29.306 Evidence from an air-oxygen practical can use removal of oxygen by a suitable reaction and a measured gas-volume fall. 00:03:29.456 --> 00:03:34.341 Control temperature and pressure and let equipment cool before comparing volumes. 00:03:37.000 --> 00:03:44.495 If 100 cubic centimetres air falls to 79 cubic centimetres after oxygen removal at the same conditions, 00:03:44.645 --> 00:03:48.101 oxygen represented about 21 cubic centimetres, 00:03:48.251 --> 00:03:50.065 or 21 percent, 00:03:50.215 --> 00:03:52.007 of the original volume. 00:03:52.157 --> 00:03:53.821 Other gases remain. 00:03:56.500 --> 00:04:02.622 Earth absorbs some incoming solar radiation and emits infrared radiation as its surface warms. 00:04:02.772 --> 00:04:06.066 Some outgoing infrared escapes directly to space. 00:04:08.733 --> 00:04:10.312 Greenhouse gases, 00:04:10.462 --> 00:04:12.190 including C O 2, 00:04:12.340 --> 00:04:14.281 methane and water vapour, 00:04:14.431 --> 00:04:19.923 absorb certain infrared wavelengths and subsequently emit radiation in different directions, 00:04:20.073 --> 00:04:22.334 including back towards the surface. 00:04:25.000 --> 00:04:29.629 This natural greenhouse effect keeps Earth warmer than it would be without these gases. 00:04:29.779 --> 00:04:34.643 It is essential to the climate, not inherently a problem to eliminate completely. 00:04:37.300 --> 00:04:46.076 Increasing greenhouse-gas concentrations enhances the effect, changing Earth's energy balance and driving warming until energy flows adjust. 00:04:46.226 --> 00:04:51.089 The mechanism is absorption and re-emission, not mirror-like reflection of all heat. 00:04:51.239 --> 00:04:56.534 Schematic energy paths: absorption and re-emission, not reflection by a solid lid. 00:05:01.200 --> 00:05:11.631 Methane has a stronger warming effect per unit mass than C O 2 over commonly used comparison periods, but the comparison depends on timescale and atmospheric lifetime. 00:05:11.781 --> 00:05:14.213 Amounts and persistence also matter. 00:05:16.867 --> 00:05:21.515 Do not confuse greenhouse warming with ozone-layer depletion or acid rain. 00:05:21.665 --> 00:05:26.948 They involve different chemistry, even where some activities contribute to more than one problem. 00:05:29.600 --> 00:05:34.349 Burning fossil fuels adds C O 2 from long-stored carbon to the atmosphere. 00:05:34.499 --> 00:05:39.458 Deforestation can release stored carbon and reduce photosynthetic uptake. 00:05:42.133 --> 00:05:46.255 Farm animals such as cattle produce methane while digesting food. 00:05:46.405 --> 00:05:56.143 Methane also escapes from fossil-fuel equipment and forms when organic material decomposes with little oxygen, for example in waterlogged soil or landfill. 00:05:58.800 --> 00:06:04.057 Climate is the pattern of conditions over long periods; weather is short-term conditions. 00:06:04.207 --> 00:06:10.531 Global warming means a long-term rise in global average temperature, not every place warming every day. 00:06:13.200 --> 00:06:22.841 Modern atmospheric measurements, surface temperature records, satellite observations, ocean measurements and ice cores provide different evidence about climate change. 00:06:25.500 --> 00:06:30.513 Ice cores contain trapped bubbles of ancient air that record past greenhouse gases. 00:06:30.663 --> 00:06:37.861 Other features of the ice provide evidence about past temperature; the gases are not simply described as dissolved in the ice. 00:06:38.011 --> 00:06:42.877 Schematic core: different evidence is used for past gas levels and temperature. 00:06:47.533 --> 00:06:54.082 Rising greenhouse gases and warming are correlated, but correlation alone is not sufficient proof of causation. 00:06:54.232 --> 00:07:00.459 Physical radiation measurements, mechanisms and multiple lines of evidence support the role of human emissions. 00:07:03.133 --> 00:07:10.685 Climate data have uncertainties, for example from instrument calibration, where measurements were taken and gaps in old records. 00:07:10.835 --> 00:07:17.927 Evidence used to estimate past conditions (proxy records) also has dating and interpretation uncertainties. 00:07:18.077 --> 00:07:22.068 Global averages use many measurements, not one local thermometer. 00:07:24.733 --> 00:07:32.626 Natural influences and variability also affect climate, but they do not explain away the evidence for human-driven recent warming. 00:07:32.776 --> 00:07:39.377 Distinguish uncertainty in exact projections from uncertainty about the basic greenhouse mechanism. 00:07:42.033 --> 00:07:49.199 Warming oceans expand, and melting land-based glaciers and ice sheets add water, contributing to sea-level rise. 00:07:49.349 --> 00:07:53.533 Melting floating ice is not the same direct addition of sea water. 00:07:53.683 --> 00:07:57.551 Floating-ice melt is not equivalent to melting a land ice sheet. 00:08:02.233 --> 00:08:05.741 Sea-level rise can increase coastal flooding and erosion. 00:08:05.891 --> 00:08:12.599 Changes in rainfall and heat can affect water supply, crop production, habitats and species distributions. 00:08:15.267 --> 00:08:21.376 Warming changes the likelihood or intensity of some extremes, including heatwaves and heavy rainfall. 00:08:21.526 --> 00:08:28.469 Do not claim every tornado, storm or single event has the same response or is solely caused by climate change. 00:08:31.133 --> 00:08:37.447 Ocean uptake of C O 2 increases acidity, lowering P H and changing carbonate chemistry. 00:08:37.597 --> 00:08:43.707 This can make it harder for some organisms to build and maintain calcium-carbonate shells or skeletons. 00:08:46.367 --> 00:08:53.416 Ocean acidification means becoming more acidic relative to before, not necessarily having P H below 7. 00:08:53.566 --> 00:08:58.984 It is a direct C O 2 chemistry effect as well as related to the broader climate problem. 00:09:01.667 --> 00:09:08.321 Heat stress can make corals lose the algae that live with them in a mutually beneficial relationship (symbiosis). 00:09:08.471 --> 00:09:11.931 This exposes their pale skeletons: coral bleaching. 00:09:12.081 --> 00:09:15.481 Bleaching and ocean acidification are different processes. 00:09:15.631 --> 00:09:20.676 Ocean warming and acidification can both affect reefs, by different mechanisms. 00:09:25.333 --> 00:09:31.119 Bleached coral can sometimes recover if stress ends, but prolonged or repeated stress can lead to death. 00:09:31.269 --> 00:09:34.925 Changes in reefs can affect many dependent organisms. 00:09:37.600 --> 00:09:44.585 Mitigation tackles the causes of climate change by reducing greenhouse-gas emissions or removing these gases. 00:09:44.735 --> 00:09:52.318 Examples include energy efficiency, low-carbon energy and transport, protecting forests and reducing methane releases. 00:09:55.000 --> 00:10:06.242 Livestock and food-system changes can lower some methane emissions, but evaluate practicality, scale and other environmental effects rather than proposing one universal solution. 00:10:08.900 --> 00:10:15.948 Carbon capture and storage aims to prevent some C O 2 reaching the atmosphere by capturing and storing it securely. 00:10:16.098 --> 00:10:20.216 Costs, energy needs, capacity and long-term leakage risks matter. 00:10:22.867 --> 00:10:26.054 Adaptation reduces the harm from climate change. 00:10:26.204 --> 00:10:32.549 Examples include flood defences, better drainage, buildings suited to hotter weather and changes to farming. 00:10:32.699 --> 00:10:37.702 Some vulnerable communities may need to move, which has social and economic costs. 00:10:37.852 --> 00:10:42.305 A response can reduce risks without eliminating the need to tackle causes. 00:10:46.967 --> 00:10:53.867 Reflecting more sunlight is a proposed solar-radiation-management approach with substantial uncertainties and risks. 00:10:54.017 --> 00:11:01.461 It does not remove C O 2 or directly solve ocean acidification and is not a simple substitute for emissions reduction. 00:11:04.133 --> 00:11:10.873 Evaluate a response using scale, cost, effectiveness, risks, timescale and environmental implications. 00:11:11.023 --> 00:11:17.107 Tradeoffs can include land use, habitats, materials and who receives benefits or bears costs. 00:11:19.767 --> 00:11:29.867 A clear explanation links activity to increased greenhouse gas, leading to stronger infrared absorption and re-emission, leading to warming and possible climate impacts. 00:11:30.017 --> 00:11:34.741 Include methane and other gases where relevant, not only carbon dioxide. 00:11:37.400 --> 00:11:40.045 That completes Earth and atmospheric science. 00:11:40.195 --> 00:11:43.541 Revisit the notes and test yourself on the revision website.