arXiv subjects

James H. Brown

Publications and source records attributed to James H. Brown.

3 recordsLinked to original sources

arXiv2018

The Central Role of Energy in the Urban Transition: Global Challenges for Sustainability

The urban transition, the increased ratio of urban to rural population globally and within countries, is a hallmark of the 21st century. Our analysis of publicly available data from the World Bank spanning several decades for ~195 countries show that across and within nations over time, per capita Gross Domestic Product (GDP), energy use, and CO2 emissions are lowest in predominantly rural countries (rural > urban pop.), increase rapidly across urbanizing countries (rural urban pop.) and are highest in the most urban countries (rural < urban pop.). These trends coincide with changes in employment by sector and gender. Rural economies are based largely on employment in the resource-extraction sector, which includes agriculture, fisheries, forestry, and mining. In urbanizing nations, male employment is predominantly in the industrial sector, including public utilities, while female employment is higher in service-based than resource-based economies. In the most urban nations, service economies predominate with some countries employing 90% of women and 65% of men in the service sector. Our analysis shows that per capita GDP, energy use, and CO2 emissions increase by over two orders of magnitude from low-income, resource-based rural countries to high-income, urbanized countries with predominantly service economies. Data from the U.S. over the past 200 years illuminate a socio-metabolic urban transition similar to that seen globally in recent decades across countries and through time. Our study suggests that increased energy demand and climate consequences of burning fossil fuels will continue to accompany a rapidly urbanizing planet posing major challenges for global sustainability.

physics.soc-ph↗

arXiv2004

Metabolic Rate Calibrates the Molecular Clock: Reconciling Molecular and Fossil Estimates of Evolutionary Divergence

Observations that rates of molecular evolution vary widely within and among lineages have cast doubts upon the existence of a single molecular clock. Differences in the timing of evolutionary events estimated from genetic and fossil evidence have raised further questions about the existence of molecular clocks and their use. Here we present a model of nucleotide substitution that combines new theory on metabolic rate with the now classic neutral theory of molecular evolution. The model quantitatively predicts rate heterogeneity, and reconciles differences in molecular- and fossil-estimated dates of evolutionary events. Model predictions are supported by extensive data from mitochondrial and nuclear genomes. By accounting for the effects of body size and temperature on metabolic rate, a single molecular clock explains heterogeneity in rates of nucleotide substitution in different genes, taxa, and thermal environments. This model suggests that there is indeed a general molecular clock, as originally proposed by Zuckerkandl and Pauling, but that it ticks at a constant substitution rate per unit mass-specific metabolic energy rather than per unit time. More generally, the model suggests that body size and temperature combine to control the overall rate of evolution through their effects on metabolism.

q-bio.PE↗

arXiv2002

Red herrings and rotten fish

A longstanding problem in biology has been the origin of pervasive quarter-power allometric scaling laws that relate many characteristics of organisms to body mass (M) across the entire spectrum of life from molecules and microbes to ecosystems and mammals. In particular, whole-organism metabolic rate, B=aM^b, where a is a taxon-dependent normalisation constant and b is approximately equal to 3/4 for both animals and plants. Recently Darveau et al. (hereafter referred to as DSAH) proposed a "multiple-causes model" for B as "the sum of multiple contributors to metabolism", B_i, which were assumed to scale as M^(b_i). They obtained for average values of b: 0.78 for the basal rate and 0.86 for the maximally active rate. In this note we show that DSAH contains serious technical, theoretical and conceptual errors, including misrepresentations of published data and of our previous work. We also show that, within experimental error, there is no empirical evidence for an increase in b during aerobic activity as suggested by DSAH. Moreover, since DSAH consider only metabolic rates of mammals and make no attempt to explain why metabolic rates for other taxa and many other attributes in diverse organisms also scale with quarter-powers (including most of their input data), their formulation is hardly the "unifying principle" they claim. These problems were not addressed in commentaries by Weibel and Burness.

physics.bio-ph↗