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Abstract. A comprehensive analysis of volatiles in the Chicxulub impact strongly supports the hypothesis that impact-generated sulfate aerosols caused over a decade of global cooling, acid rain, and disruption of ocean circulation, which contributed to the mass extinction at the Cretaceous/Tertiary (K/T) boundary. The crater size, meteoritic content of the K/T boundary clay, and impact models indicate that the Chicxulub crater was formed by a short period comet or an asteroid impact that released 0.7-3.4 x 103• ergs of energy. Impact models and experiments combined with estimates of volatiles in the projectile and target rocks predict that over 200 gigatons (Gt) each of SO2 and water vapor, and over 500 Gt of CO2, were globally distributed in the stratosphere by the impact. Additional volatiles may have been produced on a global or regional scale that formed sulfate aerosols rapidly in cooler parts of the vapor plume, causing an early, intense pulse of sulfuric acid rain. Estimates of the conversion rate of stratospheric SO2 and water vapor to sulfate aerosol, based on volcanic production of sulfate aerosols, coupled with calculations of diffusion, coagulation, and sedimentation, demonstrate that the 200 Gt stratospheric SO2 and water vapor reservoir would produce sulfate aerosols for 12 years. These sulfate aerosols caused a second pulse of acid rain that was global. Radiative transfer modeling of the aerosol clouds demonstrates (i) that if the initial rapid pulse of sulfate aerosols was global, photosynthesis may have been shut down for 6 months and (2) that for the second prolonged aerosol cloud, solar transmission dropped 80% by the end of first year and remained 50% below normal for 9 years. As a result, global average surface temperatures probably dropped between 5' and 3 IøK, suggesting that global near-freezing conditions may have been reached. Impactgenerated CO2 caused less than IøK greenhouse warming and therefore was insignificant compared to the sulfate cooling. The magnitude of sulfate cooling depends largely upon the rate of ocean mixing as surface waters cool, sink, and are replaced by upwelling of deep ocean water. This upwelling apparently drastically altered ocean stratification and circulation, which may explain the global collapse of the delta •3C gradient between surface and deep ocean waters at the K/T boundary.

Archaeological research in the Gulf Coast of Tabasco reveals the earliest record of maize cultivation in Mexico. The first farmers settled along beach ridges and lagoons of the Grijalva River delta. Pollen from cultivated Zea appears with evidence of forest clearing about 5100 calendar years B.C. (yr B.C.) [6200 (14)C years before the present (yr B.P.)]. Large Zea sp. pollen, typical of domesticated maize (Zea mays), appears about 5000 calendar yr B.C. (6000 yr B.P.). A Manihot sp. pollen grain dated to 4600 calendar yr B.C. (5800 yr B.P.) may be from domesticated manioc. About 2500 calendar yr B.C. (4000 yr B.P.), domesticated sunflower seeds and cotton pollen appear as farming expanded.

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We project populations to 2100 for the world's larger cities. Three socioeconomic scenarios with various levels of sustainability and global cooperation are evaluated, and individual "best fit" projections made for each city using global urbanization forecasts. In 2010, 757 million people resided in the 101 largest cities-11 per cent of the world's population. By the end of the century, world population is projected to range from 6.9 billion to 13.1 billion, with 15 per cent to 23 per cent of people residing in the 101 largest cities (1.6 billion to 2.3 billion). The disparate effects of socioeconomic pathways on regional distribution of the world's 101 largest cities in the 21st century are examined by changes in population rank for 2010, 2025, 2050, 2075 and 2100. Socioeconomic pathways are assessed based on their influence on the world's largest cities. Two aspects of the projections raise concerns about reliability: the unlikely degree of growth of cities suggested for Africa and the growth of cities in coastal settings (and likely global immigration). Trends and the effect of sustainable development on regional distribution of large cities throughout the 21st century are discussed. Keywords 21st century / cities / population projection / urban areas daniel Hoornweg is Associate Professor and richard Marceau chair in the Faculty of energy systems and nuclear science. Address: university of ontario Institute of technology (uoIt),

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