A mong the biggest chal- lenges the world faces today are the climate crisis and the broader issues of environmental sustainability raised in books such as Jared Diamond’s Collapse: How Societies Choose to Fail or Succeed (Viking, 2004). Part of the solution to this problem depends on climate science, breakthrough technologies, and policy changes. However, as Daniel Quinn argued in his 2002 address, “The New Renaissance” (http://ishmael.org/ Education/Writings/The_New_ Renaissance.shtml), “What we must have (and nothing less) is a whole world full of people with changed minds. Scientists with changed minds, industrialists with changed minds, school teachers with changed minds, politicians with changed minds.” He goes on to describe how each of us must find a more sustainable way to do things in businesses and at home. A commonly stated goal is to reduce world energy use to 1990 levels, thereby stabilizing atmo- spheric CO 2 emissions at 350 parts per million (J. Hansen et al., “Target Atmospheric CO2: Where Should Humanity Aim?” 18 June 2008, http://arxiv.org/abs/0804.1126v2, http://350.org). Computer scientists can help reach this goal in four ways. Two of these involve mitigating the direct negative impact of computers—their power consumption as well as the economic and social costs associated with the manufacturing, maintenance, and disposal of components. The other two relate to the indirect positive impact of computers—their ability to increase energy efficiency by changing systems and ways of being, thereby potentially reducing world emissions by as much as 15 percent by 2020, Some Computer Science Issues in Creating a Sustainable World Jennifer Mankoff, Carnegie Mellon University Robin Kravets, University of Illinois at Urbana-Champaign Eli Blevis, Indiana University Computer scientists have a role to play in combating global climate change. according to the Climate Group’s June 2008 report (www.smart2020. org); and to help provide answers to important scientific questions. COMPUTATIONAL ENERGY CONSUMPTION A Forrester Research report proj- ects the number of personal com- puters in use in the most populous countries to double to 2.25 billion by 2015 (S. Yates, “Ranking PC Growth Markets,” 10 July 2007). Embedded devices are becoming pervasive; by 2011, there will be three per person on the planet (www.microarch.org/ micro35/keynote/Agerwala.pdf). According to the Climate Group, total energy consumption by com- puters—including the power con- sumption and embodied energy of data centers, PCs and peripher- als, and networks and devices— accounted for 830 million metric tons of carbon dioxide, or 2 percent of the total world carbon footprint, in 2007. As Figure 1 shows, these figures are roughly equivalent to the total CO 2 emissions of Nigeria, Iran, and Poland, respectively. Data cen- ters alone use almost 0.5 percent of the world’s energy, and this figure is likely to quadruple by 2020. Lowering the energy cost of com- putation will depend on our ability to reduce processor cycles, communi- cation needs, and architectural inef- ficiencies. For example, according to the US Environmental Protection Agency, power adaptors consume 11 percent of US electricity, yet available design changes soon to be mandated by the Energy Star program (www. energystar.gov) can reduce their energy use by 30 percent. Hardware Hardware advances provide new opportunities for compile-time or dynamic efficiency improvement. For example, heterogeneous chip multiprocessors can achieve four to six times energy savings per instruc- tion (R. Kumar et al. “Heteroge- neous Chip Multiprocessors,” Com- puter, Nov. 2005, pp. 32-38). 102 Computer INVISIBLE COMPUTING