|Simulations of the temperature change due to all forcings. Source: USCCP|
Climate scientists have resolved a discrepancy in the rate of global average temperature increase for the surface compared with higher levels in the atmosphere, according to the National Oceanic and Atmospheric Administration (NOAA).
This discrepancy had previously been used to challenge the validity of climate models used to detect and attribute the causes of observed climate change.
The report from the US Climate Change Science Program—Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences—corrects errors that have been identified in the satellite data and other temperature observations.
Specifically, surface data had showed substantial global-average warming, while early versions of satellite and radiosonde data showed little or no warming above the surface. The team identified and corrected the errors in the satellite and radiosonde data. Furthermore, new data sets do not show such discrepancies.
Discrepancies between the data sets and the models have been reduced and our understanding of observed climate changes and their causes have increased. The evidence continues to support a substantial human impact on global temperature increases. This should constitute a valuable source of information to policymakers.—Chief Editor Thomas Karl, director of the NOAA National Climatic Data Center
The report, the first of 21 such Synthesis and Assessment (S&A) reports to be issued by the Climate Change Science Program, states that for recent decades, all current atmospheric data sets now show global-average warming that is similar to the surface warming.
The published report also states that research to detect climate change and attribute its causes using patterns of observed temperature change in space and time shows clear evidence of human influences on the climate system due to changes in greenhouse gases, aerosols and stratospheric ozone.
The observed patterns of change over the past 50 years cannot be explained by natural processes alone, nor by the effects of short-lived atmospheric constituents such as aerosols and tropospheric ozone alone.
One issue does remain, however, and that is related to the rates of warming in the tropics. Here, models and theory predict an amplification of surface warming higher in the atmosphere. However, this greater warming aloft is not evident in three of the five observational data sets used in the report. Whether this is a result of uncertainties in the observed data, flaws in climate models, or a combination of these is not yet known. Using the evidence available, the author team favors the first explanation.
|Katrina as a CAT-5 hurricane, the day before it slammed into the Gulf Coast. Credit: NOAA|
Separately, NOAA scientists reported that the region of the tropical Atlantic where many hurricanes originate has warmed by several tenths of a degree Celsius over the 20th century. New climate model simulations suggest that human activity, such as increasing greenhouse gas emissions into the atmosphere, may contribute significantly to this warming.
This very long-term increase in temperature may seem small but is comparable in magnitude to shorter time-scale, multi-decadal changes that many scientists now believe contribute strongly to an increase in hurricane activity in the Atlantic.
The challenge is to understand the relative roles of anthropogenic and natural factors in producing these temperature changes—and this study is a step in that direction—and then to determine whether and how these long-term changes in temperature could be affecting Atlantic hurricane activity.—Thomas Knutson, NOAA Geophysical Fluid Dynamics Laboratory
The region—known as the Main Development Region—extends from 10 degrees N to 20 degrees N in the area of the Cape Verde Islands, and has been identified as the origin for a large portion of major hurricanes in the tropical North Atlantic.
Ocean surface temperatures in this region warmed over the 20th century, roughly tracking the global mean, or average, but this region has greater multi-decadal variability than the global mean does when looking at long-term trends.
The climate model simulations are based on a new state-of-the-art coupled atmosphere-ocean model developed over several years at GFDL. The new simulations include improved representations of a number of environmental factors that can affect climate, such as greenhouse gases, volcanic eruptions, solar variability, land-use changes and atmospheric aerosols, very fine particulate matter in the air. More research is being conducted to improve the representation of these forcings, and of the aerosol effects in particular.
The new model simulations used current best estimates of a number of historical climate forcings to simulate climate variations over the 20th century. In the Main Development Region, the observed warming during the 20th century is simulated much more realistically in the models that include anthropogenic forcing than in models with only natural effects.
The results suggest that the century-scale warming tendency in the Main Development Region may have been caused largely by anthropogenic forcing, including increases in atmospheric greenhouse gas concentrations.
Other sources of anthropogenic forcing include aerosols and land-use changes. Examples of natural effects are volcanic emissions, long-term variability of solar radiation, and internal variability, such as the internal processes within the climate system.
The day prior to these announcements, NOAA had released the Annual Greenhouse Gas Index (AGGI), showing a global steady rise in the amount of heat-trapping greenhouse gases in the atmosphere. (Earlier post.)