FAQ - Is It Real?
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There is a lot of natural variability on interannual and decadal time scales. Given this, the question is, "How large is the change due to human activities relative to the natural variability on the same time scales." Scientists sort this out by running climate models many times and seeing what part matches observations in all cases and is therefore tied to the changes due to human activities. The natural variability, from weather or El Niņo for example, varies from one model run to the next. It is important that climate models have realistic levels of natural variability, and some models are now reproducing the El Niņo-Southern Oscillation quite realistically.
Can't most of the warming be caused by urban heat islands near surface monitoring sites?
No. Cities do produce heat that can affect temperature measurements, but climate scientists take great pains to remove the urban heat island effect from their data. It's well known that cities produce their own heat islands from the combination of concrete surfaces, less vegetation, reduced soil moisture, and exhaust from machines and cars. But scientists take this into account when assembling a data set. A NASA group removed 80% of their U.S. reporting stations from the global average based on night-time light production, leaving 200 true rural sites-still plenty-from their data. The warming pattern persisted.
Second, a U.S. National Climatic Data Center study found that from 1880 to 1998, the rural subset of their 7,500-station global record warmed slightly faster than the full record. The same study found no statistically significant urban heat island effect in 289 U.S. stations.
And finally, even though there are no cities in the oceans, warming has been measured over them too.
A climate feedback is an amplified or dampened response to an initial change in the climate system. For example, a self-reinforcing feedback (or "positive" feedback) occurs when warmer temperatures melt sea ice. The darker open water no longer reflects radiation as the ice did, but instead absorbs heat, increasing the melting, which leads to more warming, and so on. On the other hand a negative feedback would decrease warming. For example, increasing clouds in a warmer climate could cool the climate by reflecting incoming radiation. (However, clouds could also add as a positive feedback by trapping heat.) While there are a lot of uncertainties related to feedbacks from clouds and other processes, there are no negative feedbacks that we can look to with confidence to erase warming, whereas several positive feedbacks such as ice melting are expected to make warming worse.