Penn StateNASA

Conclusions

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In this module, we have covered a broad range of observations that are pertinent to recent climate change. Here is a quick recap:

Surface Temperature

Instrumental Record

This goes back to 1880; multiple analyses yield similar results, indicating a warming of about 1-1.5°C averaged over the globe in the past 150 years. The majority of the warming has occurred in the last 50 years. The map-view pattern reveals that the polar region of the northern hemisphere has warmed much more than other regions of the globe.

Satellite Measurements

Available for a much shorter time period, these results essentially confirm the analysis of the instrumental temperature record.

Ocean Warming

Sea surface temperature (SST) records from ships go back to 1850 and make up an important part of the data that provide global temperature estimates; these records are similar, though somewhat subdued in comparison to just land surface temperatures. The SST, though, represents just the skin of the oceans; to see deeper, we rely on measurements from a system of buoys that shows the oceans are slowly warming — only about 0.1 to 0.2 °C averaged over the globe during the past 50 years — but this is the temperature change in the whole upper 700 m of the oceans, which is a vast amount of water. So while the ocean has absorbed a huge amount of heat, its overall temperature has changed little.

Proxy Reconstructions

Trough the use of multiple proxies, the average global temperature has been reconstructed about 2000 years into the past. These results indicate a Medieval Warm Period (AD 950 – 1250) that was almost as warm as today, and a Little Ice Age (AD 1350 - 1850) that was more than a degree colder than today, followed by the modern warming trend.

Borehole Reconstructions

The temperature versus depth measurements from boreholes preserve a smoothed record of the history of surface temperature change; global studies of these records provide a smoothed temperature history that goes back to the year 1500. This temperature history is in good agreement with the instrumental record.

Ice

Glacier Lengths

Mountain glaciers from around the world are shrinking; some at astounding rates. The history and magnitude of melting indicate a warming history that closely matches the results from the instrumental surface temperature record.

Ice Sheet Mass

Satellite data reveal the mass changes of the two large ice sheets; both are losing mass fast (2.3e15 kg of ice in 6 yr), contributing about 8 mm to sea level rise in this short period.

Arctic Sea Ice

Submarine sonar readings and satellite measurements show that the sea ice in the Arctic Ocean is declining in thickness and in areal extent, so much so that the Northwest Passage is now open for a month or two each summer.

Sea level

Based on tide gauges, this record shows that sea level has risen about 20 cm in the past 100 years (2 mm/yr). This rise is a result of the melting of ice combined with the thermal expansion of the warmer ocean. Much more on this is Module 11.

After reviewing all of the data, here is what the leading scientific academies of the US, UK, Russia, China, France, Germany, Italy, Brazil, Japan, Canada, and India (the G8 climate change roundtable first held in Davos, Switzerland in 2005) jointly concluded:

Climate change is real. There will always be uncertainty in understanding a system as complex as the world’s climate. However, there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements, of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. This warming has already led to changes in the Earth's climate.

This is the consensus from leading scientists around the world, not politicians, journalists or business people who may stand to gain financially from taking a stand on climate change. The motivation of these scientists is to understand what the data mean and to help the broader public understand the implications of the data. At the end of the day, the data tell us that the climate is changing; the real challenge before us lies in finding ways to respond to this change through some combination of taking steps to minimize the change and finding ways to adapt to the change.