The National Academies Press

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From page 3... ... Attributing extreme weather events to climate change is challenging because these events are by definition rare and therefore hard to evaluate reliably, and are affected by patterns of natural climate variability. For instance, the biggest cause of droughts and floods around the world is the shifting of climate patterns between El Niño and La Niña events. Read the entire page →
From page 4... ... Fluctuations in sea level also occur due to changes in the amounts of water stored on land. The amount of sea level change experienced at any given location also depends on a variety of other factors, including whether regional geological processes and rebound of the land weighted down by previous ice sheets are causing the land itself to rise or sink, and whether changes in winds and currents are piling ocean water against some coasts or moving water away. Read the entire page →
From page 5... ... 15 figure 7. As CO 2 in the air has increased, there has been an increase in the CO 2 content of the surface ocean (upper box) Read the entire page →
From page 6... ... Also, as Arctic sea ice and glaciers melt, more sunlight is absorbed into the darker underlying land and ocean surfaces causing further warming and further melting of ice and snow. The biggest uncertain factor in our knowledge of feedbacks is in how the properties of clouds will change in response to climate change. Read the entire page →
From page 7... ... Both theory and direct observations have confirmed that global warming is associated with greater warming over land than oceans, moistening of the atmosphere, shifts in regional precipitation patterns and increases in extreme weather events, ocean acidification, melting glaciers, and rising sea levels (which increases the risk of coastal inundation and storm surge) Read the entire page →
From page 8... ... As we are also unable to carry out deliberate controlled experiments on Earth itself, computer models are among the most important tools used to study Earth's climate system. Climate models are based on mathematical equations that represent the best understanding of the basic laws of physics, chemistry, and biology that govern the behaviour of the atmosphere, ocean, land surface, ice, and other parts of the climate system, as well as the interactions among them. Read the entire page →
From page 9... ... Freshening of the North Atlantic from the melting of the Greenland ice sheet is however, much less intense and hence is not expected to cause abrupt changes. As another example, Arctic warming could destabilise methane (a greenhouse gas) Read the entire page →
From page 10... ... If emissions of CO 2 stopped altogether, it would take many thousands of years for atmospheric CO 2 to return to ‘pre-industrial' levels due to its very slow transfer to the deep ocean and ultimate burial in ocean sediments. Surface temperatures would stay elevated for at least a thousand years, implying extremely long-term commitment to a warmer planet due to past and current emissions, and sea level would likely continue to rise for many centuries even after temperature stopped increasing [Figure 9] Read the entire page →
From page 11... ... If all heat energy emitted from the surface passed through the atmosphere directly into space, Earth's average surface temperature would be tens of degrees colder than today. Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, act to make the surface much warmer than this, because they absorb and emit heat energy in all directions (including downwards) Read the entire page →
From page 12... ... Human activities have added greenhouse gases to the atmosphere The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased significantly since the Industrial Revolution began. In the case of carbon dioxide, the average concentration measured at the Mauna Loa Observatory in Hawaii has risen from 316 parts per million (ppm) Read the entire page →
From page 13... ... Ice core records extending back 800,000 years show that during that time, CO 2 concentrations remained within the range of 170 to 300 ppm throughout many ‘ice age' cycles -- see page B4 to learn about the ice ages -- and no concentration above 300 ppm is seen in ice core records until the past 200 years. Figure B3. Read the entire page →
From page 14... ... Climate records show a warming trend Estimating global average surface air temperature increase requires careful analysis of millions of measurements from around the world, including from land stations, ships, and satellites. Despite the many complications of synthesising such data, multiple independent teams have concluded separately and unanimously that global average surface air temperature has risen by about 0.8 °C (1.4 °F) Read the entire page →
From page 15... ... The temperature changes are relative to the global average surface temperature, averaged from 1961−1990. Source: IPCC AR5, data from the HadCRUT4 dataset (black) Read the entire page →
From page 16... ... Thus, water vapour is treated as an amplifier, and not a driver, of climate change. Higher temperatures in the polar regions melt sea ice and reduce seasonal snow cover, exposing a darker ocean and land surface that can absorb more heat, causing further warming. Read the entire page →
From page 17... ... particles in the stratosphere that reflect or absorb sunlight, leading to a short-term surface cooling lasting typically about two to three years. Over hundreds of thousands of years, slow, recurring variations in Earth's orbit around the Sun, which alter the distribution of solar energy received by Earth, have been enough to trigger the ice age cycles of the past 800,000 years. Read the entire page →
From page 18... ... Global average surface temperature change 1950 2000 2050 2100 Historical Aggressive emissions reductions "Business as usual" emissions Read the entire page →
From page 19... ... Citizens and governments can choose among several options (or a mixture of those options) in response to this information: they can change their pattern of energy production and usage in order to limit emissions of greenhouse gases and hence the magnitude of climate changes; they can wait for changes to occur and accept the losses, damage and suffering that arise; they can adapt to actual and expected changes as much as possible; or they can seek as yet unproven ‘geoengineering' solutions to counteract some of the climate changes that would otherwise occur. Read the entire page →
From page 20... ... , University of California, Berkeley ■■ Brian Hoskins FRS, Imperial College London and University of Reading ■■ John Mitchell FRS, UK Met Office ■■ Tim Palmer FRS, University of Oxford ■■ Benjamin Santer (NAS) , Lawrence Livermore National Laboratory ■■ John Shepherd FRS, University of Southampton ■■ Keith Shine FRS, University of Reading ■■ Susan Solomon (NAS) Read the entire page →
From page 21... ... 3Evidence & Causes For more detailed discussion of the topics addressed in this document (including references to the underlying original research) , see: ■■ IPCC 2013, Climate change 2013: The physical science basis. Read the entire page →

From page 3...

... Attributing extreme weather events to climate change is challenging because these events are by definition rare and therefore hard to evaluate reliably, and are affected by patterns of natural climate variability. For instance, the biggest cause of droughts and floods around the world is the shifting of climate patterns between El Niño and La Niña events.