Crowley discusses the how that 20th century temperature spikes have been a cause for the change in global climate. Observations show that the likely cause for the temperature increase has come from anthropogenic changes. Northern Hemisphere temperatures have been gathered over the last three years and analyzed by the researchers. Over the time period from 1005-1850 the data was used and it was seen that there was a warm period from 1000-1300 then a transition period from 1300-1580. Figure 1 in the paper depicts the hockey stick temperature reconstruction. The time period used in this figure ranges from 1000-1993 based on reconstructions from the Mann et al. The CL2 is the newer splice and gives a better fit than the original. The instrumental record was substituted for the proxy record because there were too few data in the CL time series and the original CL reconstruction indicated there could be variance in the 1885-1925 interval.
Figure 4 shows a two model fit for the temperature reconstruction. The CL data was spliced into the 11-point smoothed Jones et al, the Northern Hemisphere instrumental record and the smoothed Mann et al reconstruction. Jones et al instrumental record was also included for reference. The highest correlation was seen in the CL time series which was more Medieval warmth than the Mann et al. reconstruction. Figure 4A also shows the CL reconstruction with the anomalous warm interval from 1885-1925. There was also variance around 69% from 1005-1993. There were two independent lines that showed evidence of unusual nature of 20th century temperatures. The first one was that warming over the past century was unprecedented in the past 1000 years and the second one is that the same climate model can be used to explain the variability in the Northern Hemisphere temperature in the 1000-1850 interval. It shows that that there was only 25% of the 20th century temperature increase can be attributed to natural variability.
The TAR Summary for Policymakers paper addresses similar issues with climate change. The differences between the TAR summary and the Crowley paper are that the TAR summar takes a more socio-economic approach to the climate change issues rather than the experimental approach that the Crowley paper uses. The TAR summary does a good job in explaining the answer to each question while breaking it down into easier bits while the Crowley paper just uses the reconstruction of temperature to explain the variability in climate change for the last 1000 years. The TAR summary also looks to the future outlook of what can happen in the 21st century due to temperature change and sea level change. The benefits of the Crowley paper would be a more technical approach to the situation which only states facts that came from an experiment and data that was collected and analyzed. This is helpful in that we can build on this research using their same methods. The benefits of the TAR summary would be that it can be looked at as a more of an easier way for the general public to see questions on climate change broken down and explained at an easier level. The TAR summary helps in explaining how anthropogenic factors have made an impact as well as using tables and graphs to depict trends of temperature rise since 1960. The question and answer format always plays well for people who are uninformed with what is going on so the TAR summary would be able to help those people understand the severity of changing temperatures and climate change.
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