State of the Climate
Global Analysis
April 2007

National Oceanic and Atmospheric Administration

National Climatic Data Center

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Global Highlights:

Based on preliminary data, globally averaged combined land and sea surface temperature was the warmest on record for January-April year-to-date period and third warmest for April.
Global land surface temperature was warmest on record in April. Temperatures were above average in Europe, Alaska, western U.S., eastern Brazil, northwestern Africa, and most of Asia. Cooler-than-average conditions occurred in the Middle East Region and the eastern half of the contiguous U.S.
Precipitation during April 2007 was above average in the Northeast region of the contiguous U.S. and most of South America. Drier than average conditions were observed in Japan, southeastern U.S., southeastern China and most of Europe.
ENSO conditions remained in a neutral phase during April.

The data presented in this report are preliminary. Ranks and anomalies may change as more complete data are received and processed. The most current data may be accessed via the Global Surface Temperature Anomalies page.

Introduction

Temperature anomalies for April 2007 are shown on the dot maps below. The dot map, below left, provides a spatial representation of anomalies calculated from the Global Historical Climatology Network (GHCN) data set of land surface stations using a 1961-1990 base period. The dot map, below right, is a product of a merged land surface and sea surface temperature anomaly analysis developed by Smith and Reynolds (2005). Temperature anomalies with respect to the 1961-1990 mean for land and ocean are analyzed separately and then merged to form the global analysis. Additional information on this product is available.

Anomalously warm temperatures have covered much of the globe throughout the year. The January-April 2007 map of temperature anomalies shows the presence of warmer than average temperatures across all land areas, with the exception of Alaska. Additional information on Alaska temperatures are available on the U.S. National page. Warmer than average Sea Surface Temperatures (SST) occured in the equatorial Pacific, North and South Atlantic and the Indian Ocean. Cooler than average conditions were observed in the northeastern Pacific and some areas in the South Pacific.

During April, there were above average temperatures across Europe, Alaska, western U.S., eastern Brazil, northwestern Africa, and most of Asia. Cooler-than-average conditions occurred in the Middle East Region and the eastern half of the contiguous U.S. Warmer than average SSTs occurred in the Atlantic Ocean, North Indian Ocean, and the Northwestern Pacific Ocean. In the Niño regions, SST anomalies were in the range of average or slightly below average with the exception of the Niño 4 region where SST anomalies were slightly above average. These conditions are indicative of a persisting neutral ENSO phase. Please see the latest ENSO discussion for additional information.

Current month's Land SurfaceTemperature Dot map

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Current month's blended Land and sea surface Temperature Dot map

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The mean position of upper level ridges of high pressure and troughs of low pressure (depicted by positive and negative 500-millibar height anomalies on the April map) are generally reflected by areas of positive and negative temperature anomalies at the surface, respectively. For other Global products see the Climate Monitoring Global Products page.

Images of sea surface temperature conditions are available for all weeks during 2007 at the weekly SST page.

Temperature Rankings and Graphics

Effective with the February 2006 report, NCDC transitioned from the use of the Operational Global Surface Temperature Index (Quayle et al. 1999) to the blended land and ocean dataset developed by Smith and Reynolds (2005). The differences between the two methods are discussed in Smith et al. (2005).

April 2007 ranked as the third warmest April since records began in 1880 for combined global land and ocean surface temperatures. The April land surface temperature ranked warmest on record, while ocean surface temperature ranked seventh warmest in the 127-year record. For the January-April year-to-date period, the global surface temperature ranked warmest on record.

Current Month / Year-to-date

April	Anomaly	Rank	Warmest (or Next Warmest) Year on Record	Ties
Global Land Ocean Land and Ocean	+1.37°C (+2.47°F) +0.41°C (+0.74°F) +0.67°C (+1.21°F)	warmest 7th warmest 3rd warmest	2000 (+1.30°C/2.34°F) 1998 (+0.53°C/0.95°F) 1998 (+0.70°C/1.26°F)	2001
Northern Hemisphere Land Ocean Land and Ocean	+1.57°C (+2.83°F) +0.43°C (+0.77°F) +0.86°C (+1.55°F)	2nd warmest 4th warmest warmest	2000 (+1.66°C/2.99°F) 2004 (+0.53°C/0.95°F) 2005 (+0.81°C/1.46°F)	2002
Southern Hemisphere Land Ocean Land and Ocean	+0.79°C (+1.42°F) +0.40°C (+0.72°F) +0.45°C (+0.81°F)	9th warmest 9th warmest 8th warmest	1992 (+1.41°C/2.54°F) 1998 (+0.59°C/1.06°F) 1988 (+0.63°C/1.13°F)	1983

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January-April	Anomaly	Rank	Warmest (or Next Warmest) Year on Record	Ties
Global Land Ocean Land and Ocean	+1.35°C (+2.43°F) +0.44°C (+0.79°F) +0.69°C (+1.24°F)	warmest 6th warmest warmest	2002 (+1.29°C/2.32°F) 1998 (+0.53°C/0.95°F) 2002 (+0.68°C/1.22°F)
Northern Hemisphere Land Ocean Land and Ocean	+1.56°C (+2.81°F) +0.46°C (+0.83°F) +0.88°C (+1.58°F)	warmest 3rd warmest warmest	2002 (+1.54°C/2.77°F) 2004 (+0.50°C/0.90°F) 2002 (+0.83°C/1.49°F)	2005
Southern Hemisphere Land Ocean Land and Ocean	+0.72°C (+1.30°F) +0.43°C (+0.77°F) +0.47°C (+0.85°F)	4th warmest 7th warmest 6th warmest	2005 (+0.89°C/1.60°F) 1998 (+0.56°C/1.01°F) 1998 (+0.61°C/1.10°F)	2001

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The most current data may be accessed via the Global Surface Temperature Anomalies page.

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Precipitation

The maps below represent anomaly values based on the GHCN data set of land surface stations using a base period of 1961-1990. During April 2007, above average precipitation fell over areas that include the Northeast region of the contiguous U.S. and most of South America. Drier than average conditions were observed in Japan, southeastern U.S., southeastern China and most of Europe. Additional details on flooding and drought can also be found on the April Global Hazards page.

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ENSO SST Analysis

Last week of the month's ENSO condtions Map

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Sea Surface Temperature anomalies were near average or slightly cooler than average across the central and eastern equatorial Pacific, indicative of a neutral ENSO phase (shown in the adjacent animation of weekly sea surface temperature anomalies). A comprehensive summary of April 2007 ENSO conditions can be found on the ENSO monitoring page. For the latest advisory on ENSO conditions go to NOAA's Climate Prediction Center (CPC) and the CPC ENSO Diagnostic Discussion.
Images of sea surface temperature conditions are available for all weeks since 2003 at the weekly SST page.

Northern Hemisphere Snow Cover Extent

As shown in the time series to the right, mean Northern Hemisphere snow cover extent during April 2007 was below average, consistent with anomalously warm conditions that occurred during much of the winter and spring across Asia, Europe, and Alaska. April 2007 snow cover extent for the Northern Hemisphere was the third lowest extent on record, and it has been below average in 15 of the past 20 years. Mean Northern Hemisphere April snow cover extent for the 1967-2007 period of record is 31.0 million square kilometers.

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Current season's North America Snow Cover extent

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Snow cover for April 2007 across North America was below average, being the 15th lowest extent since records began in 1967. Mean North America April snow cover extent is 13.3 million square kilometers for the 1967-2007 period of record.

As depicted in the time series to the right, Eurasia's snow cover extent in April was below average and was the second lowest extent over the 41-year historical period. This was due to the anomalously warm conditions that covered much of Europe and Asia during the winter. Average Eurasian April snow cover extent is 18.0 million square kilometers for the 1967-2007 period of record.

Current season's Eurasia Snow Cover extent

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(Data were provided by Global Snow Laboratory, Rutgers University).

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Northern Hemisphere Sea Ice Extent

According to the National Snow and Ice Data Center, the Northern Hemisphere sea ice extent, which is measured from passive microwave instruments onboard NOAA satellites, was below the 1979-2000 mean. This was the least sea ice extent in April since records began in 1979. Sea ice extent for the month of April has decreased at a rate of 3.2%/decade (since satellite records began in 1979) as temperatures in the high latitude Northern Hemisphere have risen at a rate of approximately 0.37°C/decade over the same period. For further information on the Northern Hemisphere snow and ice conditions, please visit the NSIDC News page, provided by the NOAA's National Snow and Ice Data Center (NSIDC).

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Troposphere

Temperatures above the Earth's surface are measured within the lower troposphere, middle troposhere, and stratosphere using in-situ balloon-borne instruments (radiosondes) and polar-orbiting satellites (NOAA's TIROS-N). The radiosonde and the satellite records have been adjusted to remove time-dependent biases (artificialities caused by changes in radiosonde instruments and measurement practices as well as changes in satellite instruments and orbital features through time).

Lower Troposphere
Current Month / Year-to-date

These temperatures are for the lowest 8km (5 miles) of the atmosphere. Information on the UAH and RSS sources of troposphere data is available.

April	Anomaly	Rank	Warmest (or Next Warmest) Year on Record	Trend
UAH low-trop	+0.23°C/0.41°F	5th warmest	1998 (+0.78°C/1.40°F)	+0.14°C/decade
*RSS low-trop	+0.18°C/0.32°F	10th warmest	1998 (+0.90°C/1.62°F)	+0.20°C/decade

*Version 03_0

January- April	Anomaly	Rank	Warmest (or Next Warmest) Year on Record	Trend
UAH low-trop	+0.39°C/0.70°F	2nd warmest	1998 (+0.63°C/1.13°F)	+0.16°C/decade
*RSS low-trop	+0.28°C/0.50°F	7th warmest	1998 (+0.71°C/1.28°F)	+0.19°C/decade

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Mid-Troposphere
Current Month / Year-to-date

These temperatures are for the atmospheric layer centered in the mid-troposphere (approximately 2-6 miles above the Earth's surface) which also includes a portion of the lower stratosphere. (The MSU channel used to measure mid-tropospheric temperatures receives about 25 percent of its signal above 6 miles). Because the stratosphere has cooled due to increasing greenhouse gases in the troposphere and losses of ozone in the stratosphere, the stratospheric contribution to the tropospheric average, as measured from satellites, may create an artificial component of cooling to the mid-troposphere temperatures. The University of Washington (UW) versions of the UAH and RSS analyses attempt to remove the stratospheric influence from the mid-troposphere measurements, and as a result the UW versions tend to have a larger warming trend than either the UAH or RSS versions. For additional information, please see NCDC's Microwave Sounding Unit page.

The radiosonde data used in this global analysis were developed using the Lanzante, Klein, Seidel (2003) ("LKS") bias-adjusted dataset and the First Difference Method (Free et al. 2004). Additional details are available. Satellite data have been adjusted by the Global Hydrology and Climate Center at the University of Alabama in Huntsville (UAH). An independent analysis is also performed by Remote Sensing Systems (RSS) and a third analysis has been performed by Dr. Qiang Fu of the the University of Washington (UW) (Fu et al. 2004)** to remove the influence of the stratosphere on the mid-troposphere value. Global averages from radiosonde data are available from 1958 to present, while satellite measurements began in 1979.

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Radiosonde measurements indicate that for the January-April year-to-date period, temperatures in the mid-troposphere were +0.68°C (1.22°F) above average being the third warmest January-April since global measurements began in 1958. However, as shown in the table below, satellite measurements of the January-April year-to-date period for the middle troposphere varied from second to sixth warmest on record depending on the analysis method.

The global mid-troposphere temperatures were warmer than average in April 2007, as shown in the table below. Satellite measurements for April 2007 varied from seventh to eleventh warmest on record depending on the analysis method.

April	Anomaly	Rank	Warmest (or Next Warmest) Year on Record	Trend
UAH mid-trop	+0.08°C/0.14°F	11th warmest	1998 (+0.70°C/1.26°F)	+0.05°C/decade
*RSS mid-trop	+0.17°C/0.31°F	8th warmest	1998 (+0.77°C/1.39°F)	+0.12°C/decade
**UW-UAH mid-trop	+0.15°C/0.27°F	10th warmest	1998 (+0.81°C/1.46°F)	+0.12°C/decade
*UW-RSS mid-trop	+0.25°C/0.45°F	7th warmest	1998 (+0.89°C/1.60°F)	+0.18°C/decade

*Version 03_0

January- April	Anomaly	Rank	Warmest (or Next Warmest) Year on Record	Trend
UAH mid-trop	+0.21°C/+0.38°F	4th warmest	1998 (+0.57°C/1.03°F)	+0.06°C/decade
*RSS mid-trop	+0.29°C/0.52°F	6th warmest	1998 (+0.64°C/1.15°F)	+0.13°C/decade
**UW-UAH mid-trop	+0.31°C/+0.56°F	2nd warmest	1998 (+0.71°C/1.28°F)	+0.13°C/decade
*UW-RSS mid-trop	+0.38°C/+0.68°F	6th warmest	1998 (+0.76°C/1.37°F)	+0.19°C/decade
RATPAC	+0.68°C/1.22°F	3rd warmest	1998 (+0.75°C/1.35°F)	+0.15°C/decade

*Version 03_0

Stratosphere

Current Month

The table below summarizes stratospheric conditions for April 2007. On average, the stratosphere is located approximately between 10-14 miles above the Earth's surface. Over the last decade, stratospheric temperatures have been below average in part due to the depletion of ozone. The large positive anomaly in 1982 was caused by the volcanic eruption of El Chichon in Mexico, and the sharp jump in temperature in 1991 was a result of the eruption of Mt. Pinatubo in the Philippines. In both cases the temperatures returned to pre-eruption levels within two years.

April	Anomaly	Rank	Coolest Year on Record
UAH stratosphere	-0.46°C (-0.83°F)	6th coolest	1999 (-0.66°C/-1.19°F)
*RSS stratosphere	-0.36°C (-0.65°F)	7th coolest	1999 (-0.59°C/-1.06°F)

*Version 03_0

For additional details on precipitation and temperatures in April, see the Global Hazards page.

References

Christy, John R., R.W. Spencer, and W.D. Braswell, 2000: MSU Tropospheric Temperatures: Dataset Construction and Radiosonde Comparisons. J. of Atmos. and Oceanic Technology 17 1153-1170.

Free M., D.J. Seidel, J.K. Angell, J. Lanzante, I. Durre and T.C. Peterson (2005) Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC): A new dataset of large-area anomaly time series, J. Geophys. Res., 10.1029/2005JD006169.

Free, M., J.K. Angell, I. Durre, J. Lanzante, T.C. Peterson and D.J. Seidel(2004), Using first differences to reduce inhomogeneity in radiosonde temperature datasets, J. Climate, 21, 4171-4179.

Fu, Q., C.M. Johanson, S.G. Warren, and D.J. Seidel, 2004: Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends. Nature, 429, 55-58.

Lanzante, J.R., S.A. Klein, and D.J. Seidel (2003a), Temporal homogenization of monthly radiosonde temperature data. Part I: Methodology, J. Climate, 16, 224 240.

Lanzante, J.R., S.A. Klein, and D.J. Seidel (2003b), Temporal homogenization of monthly radiosonde temperature data. Part II: Trends, sensitivities, and MSU comparison, J. Climate, 16, 241 262.

Mears, Carl A., M.C. Schabel, F.J. Wentz, 2003: A Reanalysis of the MSU Channel 2 Tropospheric Temperature Record. J. Clim 16, 3650-3664.

Peterson, T.C. and R.S. Vose, 1997: An Overview of the Global Historical Climatology Network Database. Bull. Amer. Meteorol. Soc., 78, 2837-2849.

Quayle, R. G., T. C. Peterson, A. N. Basist, and C. S. Godfrey, 1999: An operational near-real-time global temperature index. Geophys. Res. Lett., 26, 333-335.

Smith, T. M., and R. W. Reynolds (2005), A global merged land air and sea surface temperature reconstruction based on historical observations (1880-1997), J. Clim., 18, 2021-2036.