Editing Space sunshade (section)

===One Fresnel lens===
Several authors have proposed dispersing light before it reaches the Earth by putting a very large lens in space, perhaps at the [[Lagrangian point#L1|L1]] point between the Earth and the Sun. This plan was proposed in 1989 by J.&nbsp;T. Early.<ref name="Early">{{Citation
|author=J. T. Early
|title=Space-Based Solar Shield To Offset Greenhouse Effect
|year=1989
|periodical=Journal of the British Interplanetary Society
|volume=42
|pages=567–569|bibcode = 1989JBIS...42..567E }}. This proposal is also discussed in footnote 23 of {{Citation
|author1=Edward Teller
|author2=Roderick Hyde
|author3=Lowell Wood
|name-list-style=amp
|title=Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change
|year=1997
|publisher=Lawrence Livermore National Laboratory
|url=https://e-reports-ext.llnl.gov/pdf/231636.pdf
|accessdate=2010-10-30
|archive-date=27 January 2016
|archive-url=https://web.archive.org/web/20160127185550/https://e-reports-ext.llnl.gov/pdf/231636.pdf
|url-status=live
}}.</ref> His design involved making a large glass (2,000 km) occulter from lunar material and placing at the L1 point.  Issues included the large amount of material needed to make the disc and also the energy to launch it to its orbit.<ref name=":0" />

In 2004, physicist and science fiction author [[Gregory Benford]] calculated that a [[concave lens|concave]] rotating [[Fresnel lens]] 1000 kilometres across, yet only a few millimeters thick, floating in space at the {{L1}} point, would reduce the solar energy reaching the Earth by approximately 0.5% to 1%.<ref name="Benford1">See [http://www.kuro5hin.org/story/2005/4/7/41932/19363 Russell Dovey, "Supervillainy: Astroengineering Global Warming] {{Webarchive|url=https://archive.today/20120804114154/http://www.kuro5hin.org/story/2005/4/7/41932/19363 |date=4 August 2012 }} and [http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=370 Bill Christensen, "Reduce Global Warming by Blocking Sunlight"] {{webarchive|url=https://web.archive.org/web/20090417153949/http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=370 |date=2009-04-17 }}.</ref>

The cost of such a lens has been disputed. At a science fiction convention in 2004, Benford estimated that it would cost about [[United States dollar|US$]]10 [[1000000000 (number)|billion]] up front, and another $10&nbsp;billion in supportive cost during its lifespan.<ref name="Benford1"/>