A HEAT wave in Chicago. Drought in Britain. Early snowmelt in Tibet. Forest fires in the Himalayas. Brush fires on Long Island. Coincidence? Or ominous indicators of global warming? As delegates from some 150 countries meet in Geneva this week to hammer out details of the United Nations agreement on climate change, experts and partisans debate the latest evidence. There's also concern about the Clinton administration's ability to reach the goal of reducing greenhouse gases to 1990 levels by the year 2000, the impact of such reductions on US jobs and the economy, and congressional cutbacks in energy and climate research. Experts stress the importance of distinguishing between short-term weather events (be it a heat wave in the United States or increased rainfall in Africa) and long-range climate trends. (Northeast drought, Page 3.) ''The temptation ... is often to attribute any of these temporal and sometimes local variations to a wider and more pervasive change in climate,'' note four scientists with the National Oceanic and Atmospheric Administration (NOAA) in a recent detailed report on US climate trends. The scientists point to a noticeable increase in ''climate extremes'' (as measured by temperature, precipitation, and drought) since 1976. ''[This] supports the notion that the climate of the US has become more extreme in recent decades,'' they write. ''Yet the magnitude and persistence of the changes are not now large enough to conclude that the climate has systematically changed to a more extreme state.'' Under the auspices of the UN, 160 nations are working to reduce the emission of greenhouse gases (such as carbon dioxide, methane, and nitrous oxide) that the international agency's scientific advisers say could disrupt weather patterns and possibly cause flooding in low-lying areas. ''The climate change problem must be addressed now,'' says Elizabeth Dowdeswell, head of the United Nations Environment Programme. ''We should not wait until atmospheric concentrations of greenhouse gases have become irreversibly high.'' At the Earth Summit in Brazil in 1992, industrial states agreed to cut emissions to 1990 levels by the year 2000. At the follow-up meeting in Berlin this April, representatives (including the US) agreed to work out legally binding arrangements for further reductions ''within specified time frames.'' While environmentalists are glad that nations have moved to require deeper reductions in greenhouse gases, they are concerned about meeting even the more modest goals. In March, Undersecretary of State Timothy Wirth said the US remains 30 percent short of that goal. ''We have between now and the year 2000 to get from here to there,'' he said. Other observers are less sanguine. ''Independent evaluations indicate that most of the climate plans issued so far [by the US and other nations] will do relatively little to slow greenhouse-gas emissions,'' asserts Worldwatch Institute senior researcher Hilary French in a July study. ''This is because they consist mainly of modest, voluntary policies.'' Shortly after he was elected, President Clinton pushed for a special tax on carbon-based energy sources like coal and oil in order to reduce their use. But Congress nixed that idea, and some lawmakers now are pushing to cut federal programs for renewable energy and climate research. The House cut NOAA's climate change research budget by 40 percent. Manufacturers and energy producers say Clinton's Climate Change Action Plan and its emphasis on voluntary steps toward energy efficiency are enough for now. They note that the US has significantly reduced its energy use per unit of economic output since the first ''oil shock'' of the early 1970s. The Global Climate Coalition (a US trade group representing utilities, railroads, manufacturers, mining companies, and oil and coal producers) notes that from 1973 to 1988 US gross national product increased 46 percent with a rise in energy consumption of just 7 percent. Since then, however, the rate of improvement in energy efficiency has fallen as the US economy grew. Those resisting drastic carbon-reduction requirements for industrialized nations also note that such countries do not present the bulk of the problem. The Intergovernmental Panel on Climate Change (IPCC), the UN's science and economic advisers on global warming, predicts that by 2025, developing countries and those in transition from centrally planned economies will be responsible for 68 percent of all energy-related carbon emissions. Insisting that the US reduce such emissions 20 percent or more below 1990 levels (as is being discussed in Geneva) ''would wreak havoc on our economy,'' says John Shlaes, executive director of the Global Climate Coalition. He estimates that cuts of this magnitude could cost an average of 600,000 jobs a year every year until the year 2010.'' President Clinton shares this concern. In a letter to Rep. John Dingell (D) of Michigan he said the US ''would not accept'' any outcome that adversely affects the US and its competitiveness.