How Journal Impact Factors Are Determined in Environmental Science

A journal impact factor (JIF) is calculated by dividing the citations received in a given year by items a journal published in the previous two years by the number of citable items it published during those same years. The calculation is the same for environmental science journals as for journals in other fields. What differs is how quickly researchers in each subfield tend to cite recent work.

For example, a soil-chemistry journal, a remote-sensing journal and an environmental-policy journal all use the same formula. Their JIFs may still differ because their citation patterns and publication mixes differ. The figure can help describe a journal’s recent citation record, but it does not measure the quality of any particular article.

The calculation

The Journal Citation Reports (JCR) from Clarivate defines the JIF as citations in a year to a journal’s items published in the two previous years, divided by the number of citable items published in those years (Journal Citation Reports: Journal Impact Factor).

In formula form:

JIF = citations in the target year to items published in the previous two years ÷ citable items published in those two years

To calculate a journal’s 2024 JIF, for instance, the numerator is citations made in 2024 to its 2022 and 2023 content. The denominator is the number of citable items it published in 2022 and 2023. The result is reported in a later JCR release.

One example

Imagine an environmental science journal published 400 citable items in 2022 and 600 in 2023. In 2024, those items received 5,000 citations counted by Clarivate.

• Numerator: 5,000 citations
• Denominator: 400 + 600 = 1,000 citable items
• JIF: 5,000 ÷ 1,000 = 5.0

This example uses round figures to show the arithmetic. A real JIF depends on Clarivate’s records and classification of the journal’s content.

What counts in the calculation

The numerator: citations in the target year

The numerator counts citations made during the target year to the journal’s content from the two preceding years. For a 2024 JIF, that means citations made in 2024 to items published in 2022 and 2023.

The metric depends on the records and coverage used by Clarivate. A citation to a paper in a source outside its indexed data may not be included in the calculation. The exact records used for an individual journal’s JIF should be checked in JCR.

The denominator: citable items

The denominator is not necessarily every item a journal published. It counts items classified as citable under the JIF method. Research articles and reviews are commonly included; other content may be treated differently. The classification can affect the ratio because a change in the denominator changes the result even if the citation count stays the same.

This is one reason to avoid treating the JIF as a simple count of how well every piece of a journal’s content performs. It is a ratio based on a defined set of citations and a defined set of items. For details about a particular journal, consult the JCR record rather than assuming that every item on its website is counted in the same way.

Which journals receive a JIF

Clarivate’s JIF is reported for journals included in the Web of Science Core Collection. Clarivate describes the Core Collection as its curated index of scholarly publications (Web of Science Core Collection). Information about JCR and its journal metrics is available from Clarivate (Clarivate Journal Citation Reports).

A publisher may display a different journal metric or an unofficial figure. Check its source and method before calling it a JIF. A metric with a similar name may use different data or a different calculation.

Why environmental science journals can differ

The formula does not change by discipline, but citation patterns do. Environmental science includes fields with different research questions, publication practices and citation timelines. A journal focused on one topic may publish work that attracts citations quickly, while another may publish research that takes longer to enter wider use.

The two-year publication window is therefore a limited view of a journal’s citation record. Work on long-term observations or slow-moving research questions may continue to attract citations after it falls outside that window. The JIF will not include those later citations in a calculation tied to the paper’s publication year.

Reviews can also affect a journal’s overall figure. A review that brings together research on a topic such as microplastics in freshwater systems may receive citations from many later papers. That can influence a journal’s JIF, though the metric does not show how citations are distributed among the journal’s individual items.

These features make comparisons across environmental science subfields difficult. A raw figure does not account for differences in citation pace or research focus. A comparison is more useful when it involves journals in a relevant subject category and considers what each journal publishes.

How to read a JIF responsibly

A JIF describes a journal’s citation record over a defined period. It does not establish the quality of an individual paper, the soundness of its methods or the importance of its conclusions. A journal with a higher JIF can publish papers with very different levels of influence and quality.

When comparing journals, check the subject category and the scope of each title. A journal’s category position can add context to its raw JIF, but category rankings are still comparisons within a particular classification system. Make sure the journals are listed in a relevant category and that you are comparing the same metric.

For a paper you are considering, read the paper itself. Look at its methods, data and conclusions. If citation activity matters to your assessment, check citations to that paper directly rather than inferring its reception from the JIF of the journal that published it.

JCR includes other journal metrics, such as the five-year impact factor and the Immediacy Index. They answer different questions from the standard two-year JIF. Compare like with like, and check the metric name before interpreting a number.

Key Takeaways

• The JIF divides citations in a target year to a journal’s items from the previous two years by the number of citable items it published in those years (Journal Citation Reports: Journal Impact Factor).
• Clarivate reports JIFs through Journal Citation Reports for journals covered by its Web of Science Core Collection.
• The denominator counts citable items, not necessarily everything a journal publishes.
• Environmental science journals can have different JIFs because citation patterns and timelines vary across subfields.
• A JIF describes a journal’s recent citation record. It does not rate an individual paper or replace reading the research.