Sir — Authorship is the currency ofmodern science, the measure of one’scontribution to the literature. Yet while the contents of an article are held to strictscientific standards, the recognition ofauthorship is far less so.

With no accepted system to guide theprocess by which authors are recognizedfor their individual contributions, and withthe numbers of authors per article steadilyincreasing, the incidence of authorshipdisputes and abuse is rising, as EugenTarnow noted in Correspondence (“When extra authors get in on the act”Nature 398, 657; 1999).

Certain medical journals now requireauthorship declarations, although these are always qualitative. We suggest aquantitative method for evaluatingauthorship based on four categories ofcontribution: conception and design, datacollection, data analysis and conclusions,and manuscript preparation. Each authorwould claim their percentage share of thetotal credit in each of the four categories.The least that one could contribute to a paper would be 10% within a singlecategory, placing a theoretical limit of40 on the total number of authors.

Authors would usually be listed indescending order of total contribution

across all four categories. When equal sumsare reported, the listing could be madealphabetically, and for large collaborations,either groups or individuals may declaretheir contributions. Journals would be freeto choose their own categories, but thesewould ideally be uniform across all papers.

The percentage share could be indicated as in the following example,reflecting a typical distribution in a student–mentor relationship in which the student,S, is responsible for 20% of conception and design, 90% of data collection,80% of data analysis and conclusions and20% of manuscript preparation:S20,90,80,20, M80,10,20,80.

The explicit nature of this system,dubbed Quantitative Uniform AuthorshipDeclaration (QUAD), permits the reader toidentify who contributed what, rapidly andeasily. It should also discourage distortionsof authorship by co-authors in positions ofpower and help reduce the numbers ofhonorary authorships.

Without external monitoring, it wouldbe impossible to guarantee that all theauthors have described their contributionsaccurately. However, the quantitative andtransparent nature of QUAD should helpreduce such behaviour.

Journals that have introduced

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authorship declarations have seen onlypartial adoption by authors, but we hopethat many more journals and authors willwelcome the advantages offered by QUAD.Ultimately, this system will prove moreuseful than existing ad hoc methods whenscientists and administrators are faced withfunding and hiring decisions.Justus V. Verhagen50,30,30,30*,Karin J. Wallace30,30,30,10†,Stephan C. Collins20,40,20,10‡,Thomas R. Scott0,0,20,50§*Department of Experimental Psychology,University of Oxford, South Parks Road,Oxford OX1 3UD, UK†Wellcome Trust Centre for Human Genetics,University of Oxford, Roosevelt Drive,Oxford OX3 7BN, UK‡The Wellcome Trust Sanger Institute,Hinxton, Cambridge CB10 1SA, UK§College of Sciences, San Diego State University,5500 Campanile Drive, San Diego,California 92182-1010, USA

Nature’s authorship policy is that authorsare strongly encouraged to include astatement in the Acknowledgements tospecify the actual contribution of each co-author. See www.nature.com/nature/submit/policies/index.html#2 — Editor,Correspondence

Tidewater glaciers moveat their own paceSir — Arctic warming is receiving muchscientific scrutiny and public interest, andyour News Feature about Alaska’s climate“Too hot to handle” (Nature 425, 338–339;2003) gives a generally accurate overviewof the reasons why the Arctic deserves our continued attention. However, one ongoing event in your list of directconsequences of recent Arctic warming,the rapid retreat of Columbia Glacier,should not be there.

Columbia Glacier is one of manytidewater glaciers — glaciers that terminatein the ocean but do not float — that fringeAlaska’s central and southern coasts.They are well-known to advance andretreat out of synchrony with land-terminating glaciers, and even with eachother. Climate appears to control tidewaterglacier advance and retreat only on long(millennial) timescales; shorter annual,decadal, and even century-to-centurycomparisons do not show a closerelationship between tidewater glacieractivity and climate.

Early European explorations of Alaskaat the end of the eighteenth centuryshowed that tidewater glaciers in southernAlaska were fully extended, with terminireaching all the way to the coast, butduring the nineteenth and early twentiethcenturies these glaciers rapidly retreated,exposing such long and deep fjords asJohns Hopkins Inlet and DisenchantmentBay. Columbia Glacier attained itsadvanced position between the fifteenthand nineteenth centuries, and only sincethe early 1980s has it started its retreat.

The end of Columbia Glacier hasretreated roughly 13 kilometres from itspre-1980 position, and continues to retreatat roughly half a kilometre per year whiledischarging some 11 cubic kilometres ofice per year into Prince William Sound.But as Columbia Glacier retreats, HubbardGlacier, 400 kilometres to the southeast in Disenchantment Bay, has beenadvancing since 1980.

None of this is to say that tidewaterglacier behaviour is irrelevant to present-day concerns about climate change.Tidewater glaciers are affected by climatechange on long timescales (primarilythrough accumulation of mass by snowfall

and loss by melt), but are even morestrongly affected on short timescales by the ocean, through the influence of heatadvection and forces of hydrostatic water pressure. These factors add greatcomplexity to tidewater glacier dynamicsand result in nonlinear and irreversibleresponses to climate, which on long time-scales does indeed control all glaciergrowth and contraction.

Arctic warming is very real, but thecomplicated interaction between tide-water glaciers, oceans and climate — and the likelihood that tidewater glacierretreat may not be caused directly by the current climate — should be morewidely recognized.W. T. Pfeffer Department of Civil, Environmental andArchitectural Engineering and Institute of Arctic and Alpine Research,Campus Box 450, University of Colorado,Boulder, Colorado 80309-0450, USA

correspondenceContributions to Correspondence may be submitted to corres@nature.com. They should be no longerthan 500 words, and ideally shorter. Published contributions are edited.

QUAD system offers fair shares to all authorsShowing who did what could solve the problem of authorship abuse and disputes.

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