time.". His premise that grassland reflects more sun, keeping temperatures lower, is, however, applicable only in arid regions. A well-watered lawn, for example, is as green as a tree, but absorbs far less . Deciduous trees also have the advantage of providing shade in the summer and sunlight in the winter; so these trees, when planted close to houses, can be utilized to help increase energy efficiency of these houses. This study remains controversial and criticized for assuming dark colored trees might replace the frozen, white tundra in the upper northern hemisphere. Regular tree planting projects typically take place on lands that are only slightly different in color. The warming impact was also measured over hundreds of years, rather than a 30- to 70-year time horizon most climate experts believe we have to fix climate change. Furthermore, the described warming effect (of temperate and boreal latitude forest) is only apparent once the trees have grown to create a dense 'close canopy', and it is at precisely this point that trees grown for offset purposes should be harvested and their absorbed carbon fixed for the long-term as timber. Costs While the benefits of tree planting are subject to debate, the costs are low compared to many other mitigation options. The IPCC has concluded that "The mitigation costs through forestry can be quite modest (US$0.1–US$20 / metric ton carbon dioxide) in some tropical developing countries ... The costs of biological mitigation, therefore, are low compared to those of many other alternative measures". The cost-effectiveness of tropical reforestation is due not only to growth rate, but also to farmers from tropical developing countries who voluntarily plant and nurture tree species which can improve the productivity of their lands. As little as US$90 will plant 900 trees, enough to annually remove as much carbon dioxide as is annually generated by the fossil-fuel usage of an average United States resident. Types of trees planted The type of tree planted may have great influence on the environmental outcomes. It is often much more profitable to outside interests to plant fast-growing species, such as eucalyptus, casuarina or pine (e.g., Pinus radiata or Pinus caribaea), even though the environmental and biodiversity benefits of such monoculture plantations are not comparable to native forest, and such offset projects are frequently objects of controversy. To promote the growth of native ecosystems, many environmentalists advocate only indigenous trees be planted. A practical