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Now research shows the night is getting even brighter. From 2012 to 2016 the earth's artificially lit area expanded by an estimated 2.2 percent a year (map), according to a study published last November in Science Advances. Even that increase may understate the problem, however. The measurement excludes light from most of the energy-efficient LED lamps that have been replacing sodium-vapor technology in cities all over the world, says lead study author Christopher Kyba, a postdoctoral researcher at the German Research Center for Geosciences in Potsdam.

I can’t find any sentence that is in contrast with the sentence attached with however?

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"However" here refers to certain unspoken assumptions that the author is expecting the reader to understand. If this is the beginning of the article, then a few introductory sentences along the following lines are omitted:

  1. Artificial lighting is a problem that we would like to alleviate.
  2. We cannot address this problem without accurately measuring it.
  3. Studies have been performed to quantify the effect.

Therefore, "however" refers to the contrast between the accurate evaluation that we hope for and the underestimation that results from omitting light from certain energy-efficient LED lamps, as explained in the rest of the paragraph.

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Research shows the night is getting brighter. However, the 2.2% annual increase in brightness between 2012 and 2016, as presented by a recent study, may understate the problem.

The contrast is between the stated rate and the actual rate of increase.

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If you like "rules", you will find some editors who have a rule that a sentence may not begin with "however." Such editors would always replace "Today was hot. However, the sun was not shining" with "Today was hot. The sun, however, was not shining." The reason for the 'rule' is that if you are telling a complicated story you may find yourself beginning every alternate sentence after the first with "However,". "A. However, B. C. However, D" and so on.

The point of the word "however" is to contrast a point about to be made with one that was made immediately previously. In the example quoted the "however" signals the point that 2.2% might be an understatement. Not as a point of grammar, I would want to know, however, why 2.2% is necessarily bad. Bare figures like that prove nothing.

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