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Looking at the following temperature reconstructions i have two questions (source: wikipedia):

enter image description here

  1. Why is the y-axis centered around -0.2 °C?

  2. Why are the consequences of the current temperature increase by +0.4 °C considered so "catastrophic" when the -0.8 °C during the little ice age was not? Maybe i am mistaken but looking at the chart the current +0.4 °C doesn't seem so much more varying from the median that the -0.8 °C do.

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    $\begingroup$ when the -0.8 °C during the little ice age was not Note that there probably is lot of bias in that observation: we have far less information about weather effects in that period than we do about current (or even future) effects $\endgroup$
    – Jan Doggen
    Commented Apr 11, 2017 at 11:17
  • $\begingroup$ I think the question could use a little clarification. In relation to this graph, where is the claim that a +0.4 °C anomaly (in 2004, 13 years ago) is catastrophic? That's not made on the linked page. According to the National Centers for Environmental Information, the average global temperature for Feb. 2017 was 0.98 °C above the 20th century average: ncdc.noaa.gov/sotc/global/201702 $\endgroup$
    – jeffronicus
    Commented Apr 12, 2017 at 4:00
  • $\begingroup$ becasue the current temprature increase is not expected to stop at 0.4°C, the temprature follows CO2 and methane concentration fairly closely and our current levels for both are way higher than we have ever seen before. $\endgroup$
    – John
    Commented Apr 12, 2017 at 13:36

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To be sure about why it appears to be centered around -0.2°C, you'd have to go to the original source of the graph. However, as an educated guess, I would imagine that they are showing the anomaly compared to some recent period, such as 1961-1990 or 1971-2000, as is commonly done. From your Wikimedia Commons link, this figure appears to be original work by a Wikipedia author. The link does not appear to state this, so this is rather sloppy and perhaps illustrates why it's preferable to obtain information from peer reviewed sources.

If the temperature increase stays at +0.4°C, it is not catastrophic. However, considering the greenhouse gases (GHG) we have already pumped into the atmosphere, temperature increase will not stay at +0.4°C. With currently projected increases in GHG emissions, the global average temperature increase may range from anything between +1.5°C and +6.5°C, with some regions exhibiting far larger temperature increases than that. At such scales, the entire climate system might enter a different state, which means many regions will have climates very unlike before. And then I'm not even mentioning the problems associated with sea level rise yet.

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    $\begingroup$ I'd think there's an even simpler explanation for the -0.2°C centering: the author fed the data into a plotting program such as gnuplot, and that's what the autoscaling function came up with. $\endgroup$
    – jamesqf
    Commented Apr 11, 2017 at 19:57
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"Why is the y-axis centered around -0.2 °C?"

It's not.

The anomaly graph is actually centered at 0 degrees and a particular date, probably the late 1970s, as Gerrit alludes to. Where the axes cross is unimportant. The bottom end of this plot has been expanded to show negative anomalies (everything BELOW 0 degrees) as well as the positive anomalies (everything ABOVE 0 degrees).

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